GPL FE (GNU GPL With Font Exception)

The GNU General Public Licenses (GNU GPL or simply GPL) are a series of widely used free software licenses, or ''copyleft'' licenses, that guarantee end users the freedom to run, study, share, or modify the software. The GPL was the first copyleft license available for general use. It was originally written by Richard Stallman, the founder of the Free Software Foundation (FSF), for the GNU Project. The license grants the recipients of a computer program the rights of the Free Software Definition. The licenses in the GPL series are all copyleft licenses, which means that any derivative work must be distributed under the same or equivalent license terms. The GPL is more restrictive than the GNU Lesser General Public License, and even more distinct from the more widely used permissive software licenses such as BSD, MIT, and Apache.

Historically, the GPL license family has been one of the most popular software licenses in the free and open-source software (FOSS) domain. Prominent free software programs licensed under the GPL include the Linux operating system kernel and the GNU Compiler Collection (GCC). David A. Wheeler argues that the copyleft provided by the GPL was crucial to the success of Linux-based systems, giving the contributing programmers some assurance that their work would benefit the world and remain free, rather than being potentially exploited by software companies who would not be required to give back to the community.

In 2007, the third version of the license (GPLv3) was released to address some perceived problems with the second version (GPLv2), which were discovered during the latter's long-term use.

To keep the license current, the GPL includes an optional "any later version" clause, which allows users to choose between two options—the original terms or the terms in new versions as updated by the FSF. Software projects licensed with the optional "or later" clause include the GNU Project, but projects such as the Linux kernel are licensed under GPLv2 only. The "or any later version" clause is sometimes known as a ''lifeboat clause,'' since it allows combinations of different versions of GPL-licensed software to maintain compatibility.

Usage of the GPL has steadily declined since the 2010s, particularly because of the complexities mentioned above, as well as a perception that the license restrains the modern open source domain from growth and commercialization.

History

The original GPL was written by Richard Stallman in 1989, for use with programs released as part of the GNU Project. The license was based on unifying similar licenses used for early versions of the GNU Emacs text editor, the GNU Debugger, and the GNU C Compiler. These licenses contained provisions similar to the modern GPL, but they were specific to each program, which rendered them incompatible despite being the same license. Stallman's goal was to produce a single license that could be used for any project, thereby enabling many projects to share code.

The second version of the license, GPLv2, was released in 1991. During the subsequent 15 years, members of the free software community became concerned about specific problems in the GPLv2 license, which could allow a person to exploit GPL-licensed software in ways contrary to the license's intent. These problems included the following:

* tivoization—the inclusion of GPL-licensed software in hardware that prevents users from running modified versions of the software (on that hardware)
* compatibility issues similar to those of the AGPL (v1)
* patent deals between Microsoft and distributors of free and open-source software, which some people viewed as an attempt to use patents as a weapon against the free software community

Version 3 of the GPL was developed as an attempt to address the concerns mentioned above; it was officially released on 29 June 2007.

Version 1

Version 1 of the GNU GPL, released on 25 February 1989, was written to protect against the two main methods by which software distributors restricted the freedoms that define free software.

The first method is publishing binary files that are only executable, but not readable or modifiable by humans. To prevent this restriction, the GPLv1 states that copying and distributing copies of any portion of the program must also make the human-readable source code available under the same licensing terms.

The second method is adding restrictions, either directly to the license or by combining the software with other software having different restrictions on distribution. The union of two such sets of restrictions would apply to the combined work, thereby adding unacceptable constrictions. To prevent this situation, the GPLv1 states that modified versions, as a whole, must be distributed under the terms of GPLv1. As a result, software distributed under the terms of the GPLv1 could be combined with software distributed under more permissive terms, since this combination would not change the terms under which the whole could be distributed. However, software distributed under GPLv1 could not be combined with software distributed under a more restrictive license, since this combination would conflict with the requirement that the whole be distributable under the terms of GPLv1.

Version 2

According to Richard Stallman, the major change in version 2 of the GPL was the "Liberty or Death" clause—Section 7. The section states that licensees may distribute a GPL-covered work ''only'' if they can satisfy all of the license's obligations, despite any other legal obligations they might have. In other words, the obligations of the license may not be severed due to conflicting obligations. This provision is intended to discourage any party from using a patent infringement claim or other litigation to impair users' freedom under the license.

By 1990, it became apparent that a less restrictive license would be strategically useful for two kinds of libraries: the C standard library and software libraries that handled the same tasks as existing proprietary libraries. When the GPLv2 was released in June 1991, a second license was introduced at the same time; this was the GNU Library General Public License (LGPL), numbered as version 2 to show that the two licenses were complementary. The version numbers diverged in 1999 when version 2.1 of the LGPL was released, renamed as the GNU Lesser General Public License to reflect its role in the philosophy. The GPLv2 was updated to refer to the new name of the LGPL, but the GPL's version number remained the same. As a result, the original GPLv2 was not recognized by the Software Package Data Exchange (SPDX).

The GPL includes instructions to specify "version 2 of the License, or (at your option) any later version", to allow the flexible use of either version 2 or 3, but some developers change these instructions to specify "version 2" only.

Version 3

In late 2005, the Free Software Foundation (FSF) announced work on version 3 of the GPL. On 16 January 2006, the first "discussion draft" of GPLv3 was published, and public consultation began. The official GPLv3 was released by the FSF on 29 June 2007. GPLv3 was written by Richard Stallman, with legal counsel from Eben Moglen and Richard Fontana from the Software Freedom Law Center.

According to Stallman, the most important changes involved software patents, free software license compatibility, the definition of "source code", and hardware restrictions on software modifications (such as tivoization).Interview with Richard Stallman
, Free Software Magazine, 23 January 2008. Other changes involved internationalization, the handling of license violations, and the granting of additional permissions by the copyright holder. The term ''software propagation'' was explicitly defined as the copying and duplicating of software.

The public consultation process was coordinated by the Free Software Foundation with assistance from Software Freedom Law Center, Free Software Foundation Europe, and other free software groups. Comments were collected from the public via the gplv3.fsf.org web portal, using purpose-written software called stet. By the end of the comment period, a total of 2,636 comments had been submitted.

The third draft of GPLv3 was released on 28 March 2007. This draft included language intended to prevent patent-related agreements such as the controversial Microsoft-Novell agreement; the draft also restricted the anti-tivoization clauses to legal definitions of "user" and "consumer product". The draft also explicitly removed the section on "Geographical Limitations", the likely removal of this section having been announced at the launch of the public consultation.

The fourth and final discussion draft was released on 31 May 2007. It introduced compatibility with Apache License version 2.0 (since prior versions are incompatible); clarified the role of external contractors; and made an exception to avoid perceived problems with an agreement in the Microsoft–Novell style, stating in Section 11, paragraph 6, as follows:

This exception aimed to make such future deals ineffective. The license was also intended to cause Microsoft to take action—to extend the patent licenses it granted to Novell customers for the use of GPLv3 software to ''all'' users of that GPLv3 software; this extension was possible only if Microsoft was legally a "conveyor" of the GPLv3 software.

Early drafts of GPLv3 also let licensors add a requirement like the AGPL that would have plugged a loophole in the GPL regarding application service providers.List of free-software licences on the FSF website
"We recommend that developers consider using the GNU AGPL for any software which will commonly be run over a network." The freedom to run, study, and share the source code and guarantee copyleft protections is somewhat ambiguous in the context of web services. However, concerns were expressed about the administrative costs of checking code for the additional requirements in the GPLv3 drafts; it was ultimately decided to keep the GPL and the AGPL separated.

Others, notably high-profile Linux kernel developers such as Linus Torvalds, Greg Kroah-Hartman, and Andrew Morton, commented to the mass media and made public statements about their objections to parts of the GPLv3 drafts. The kernel developers disapproved of GPLv3 draft clauses regarding DRM/tivoization, patents, and "additional restrictions", and warned of a Balkanisation of the "Open Source Universe". Linus Torvalds, who decided not to adopt the GPLv3 for the Linux kernel, reiterated his criticism several years later.

GPLv3 improved compatibility with several free software licenses such as the Apache License, version 2.0, and the GNU Affero General Public License, which GPLv2 could not be combined with. However, GPLv3 software could only be combined and share code with GPLv2 software if the GPLv2 license used had the optional "or later" clause and the software was upgraded to GPLv3. While the "GPLv2 or any later version" clause is considered by FSF as the most common form of licensing GPLv2 software, Toybox developer Rob Landley described it as a ''lifeboat clause''. Software projects licensed with the optional "or later" clause include Joomla and the GNU Project, while a prominent example without the clause is the Linux kernel.

The final version of the license text was published on 29 June 2007.

Terms and conditions

The terms and conditions of the GPL must be made available to anybody receiving a copy of a work that has a GPL license applied to it ("the licensee"). Any licensee who adheres to the terms and conditions is given permission to modify the work, as well as to copy and redistribute the work or any derivative version. The licensee is allowed to charge a fee for this service or do this free of charge. This latter point distinguishes the GPL from software licenses that prohibit commercial redistribution. The FSF argues that free software should not place restrictions on commercial use, and the GPL explicitly states that GPL works may be sold at any price.

The GPL additionally states that a distributor may not impose "further restrictions on the rights granted by the GPL". This forbids activities such as distributing the software under a non-disclosure agreement or contract.

The fourth section of the GPLv2 and the seventh section the GPLv3 require that programs distributed as pre-compiled binaries be accompanied by a copy of the source code, a written offer to distribute the source code via the same mechanism as the pre-compiled binary, or the written offer to obtain the source code that the user got when they received the pre-compiled binary under the GPL. The second section of the GPLv2 and the fifth section of the GPLv3 also require distributing the license along with the program. The GPLv3 allows making the source code available in additional ways in fulfillment of the seventh section. These include downloading source code from an adjacent network server or by peer-to-peer transmission, provided that is how the compiled code was available and there are "clear directions" on where to find the source code.

The FSF does not hold the copyright for a work released under the GPL unless an author explicitly assigns copyrights to the FSF (which seldom happens except for programs that are part of the GNU Project). Only the individual copyright holders have the authority to sue when a license violation is suspected.

Use of licensed software

Software under the GPL may be run for all purposes, including commercial purposes and even as a tool for creating proprietary software, such as when using GPL-licensed compilers. Users or companies who distribute GPL-licensed works (e.g. software), may charge a fee for copies or give them free of charge. This distinguishes the GPL from shareware software licenses that allow copying for personal use but prohibit commercial distribution or proprietary licenses where copying is prohibited by copyright law. The FSF argues that freedom-respecting free software should also not restrict commercial use and distribution (including redistribution):

However, software running as an application program under a GPL-licensed operating system such as Linux is not required to be licensed under GPL or to be distributed with source-code availability—the licensing depends only on the used libraries and software components and not on the underlying platform. For example, if a program consists only of original source code, or is combined with source code from other software components, then the custom software components need not be licensed under GPL and need not make their source code available; even if the underlying operating system used is licensed under the GPL, applications running on it are not considered derivative works. Only if GPL-licensed parts are used in a program (and the program is distributed), then all other source code of the program needs to be made available under the same license terms. The GNU ''Lesser'' General Public License (LGPL) was created to have a weaker copyleft than the GPL, in that it does not require custom-developed source code (distinct from the LGPL-licensed parts) to be made available under the same license terms.

The fifth section of the GPLv3 states that no GPL-licensed code shall be considered an effective "technical protection measure" as defined by Article 11 of the WIPO Copyright Treaty, and that those who convey the work waive all legal power to prohibit circumvention of the technical protection measure "to the extent such circumvention is effected by exercising rights under this License with respect to the covered work". This means that users cannot be held liable for circumventing DRM implemented using GPLv3-licensed code under laws such as the US Digital Millennium Copyright Act (DMCA).

Copyleft

The distribution rights granted by the GPL for modified versions of the work are not unconditional. When someone distributes a GPL-licensed work plus their own modifications, the requirements for distributing the whole work cannot be any greater than the requirements that are in the GPL. This requirement is known as copyleft. It earns its legal power from the use of copyright on software programs.

Because a GPL work is copyrighted, a licensee has no right to redistribute it, not even in modified form (barring fair use), except under the terms of the license. One is only required to adhere to the terms of the GPL if one wishes to exercise rights normally restricted by copyright law, such as redistribution. Conversely, if one distributes copies of the work without abiding by the terms of the GPL (for instance, by keeping the source code secret), they can be sued by the original author under copyright law.

Copyright law has historically been used to prevent distribution of work by parties not authorized by the creator. Copyleft uses the same copyright laws to accomplish a very different goal. It grants rights to distribution to all parties insofar as they provide the same rights to subsequent ones, and they to the next, etc. In this way, the GPL and other copyleft licenses attempt to enforce libre access to the work and all derivatives.

Many distributors of GPL-licensed programs bundle the source code with the executables. An alternative method of satisfying the copyleft is to provide a written offer to provide the source code on a physical medium (such as a CD) upon request. In practice, many GPL-licensed programs are distributed over the Internet, and the source code is made available over FTP or HTTP. For Internet distribution, this complies with the license.

Copyleft applies only when a person seeks to redistribute the program. Developers may make private modified versions with no obligation to divulge the modifications, as long as they do not distribute the modified software to anyone else. Copyleft applies only to the software, and not to its output (unless that output is itself a derivative work of the program). For example, a public web portal running a modified derivative of a GPL-licensed content management system is not required to distribute its changes to the underlying software, because the modified web portal is not being redistributed but rather hosted, and also because the web portal output is also not a derivative work of the GPL-licensed content management system.

There has been debate on whether it is a violation of the GPLv1 to release the source code in obfuscated form, such as in cases in which the author is less willing to make the source code available. The consensus was that while unethical, it was not considered a violation. The issue was clarified when the license was altered with v2 to require that the "preferred" version of the source code be made available.

License versus contract

The GPL was designed as a license, rather than a contract. In some common law jurisdictions, the legal distinction between a license and a contract is an important one: contracts are enforceable by contract law, whereas licenses are enforced under copyright law. However, this distinction is not useful in the many jurisdictions where there are no differences between contracts and licenses, such as civil law systems.

Those who do not accept the GPL's terms and conditions do not have permission, under copyright law, to copy or distribute GPL-licensed software or derivative works. However, if they do not redistribute the GPL-licensed program, they may still use the software within their organization however they like, and works (including programs) constructed by the use of the program are not required to be covered by this license.

Software developer Allison Randal argued that the GPLv3 as a license is unnecessarily confusing for lay readers, and could be simplified while retaining the same conditions and legal force.

In April 2017, a US federal court ruled that an open-source license is an enforceable contract.

In October 2021 Software Freedom Conservancy sued Vizio over breach of contract as an end user to request source code for Vizio's TVs. A federal judge has ruled in the interim that the GPL is an enforceable contract by end users as well as a license for copyright holders.

Derivations

The text of the GPL is itself copyrighted, and the copyright is held by the Free Software Foundation.

The FSF permits people to create new licenses based on the GPL, as long as the derived licenses do not use the GPL preamble without permission. This is discouraged, however, since such a license might be incompatible with the GPL and causes a perceived license proliferation.

Other licenses created by the GNU Project include the GNU Lesser General Public License, GNU Free Documentation License, and GNU Affero General Public License.

The text of the GPL is not itself under the GPL. The license's copyright disallows modification of the license. Copying and distributing the license is allowed since the GPL requires recipients to get "a copy of this License along with the Program." According to the GPL FAQ, anyone can create a new license using a modified version of the GPL as long as they use a different name for the license, do not mention "GNU," and remove the preamble. However, the preamble can be used in a modified license if permission to use it is obtained from the Free Software Foundation (FSF).

Linking and derived works

Libraries

According to the FSF, "The GPL does not require you to release your modified version or any part of it. You are free to make modifications and use them privately, without ever releasing them." However, if one releases a GPL-licensed entity to the public, there is an issue regarding linking: namely, whether a proprietary program that uses a GPL library is in violation of the GPL.

This key dispute is whether non-GPL software can legally statically link or dynamically link to GPL libraries. Different opinions exist on this issue. The GPL is clear in requiring that all derivative works of code under the GPL must themselves be under the GPL. Ambiguity arises with regard to using GPL libraries and bundling GPL software into a larger package (perhaps mixed into a binary via static linking). This is ultimately a question not of the GPL ''per se'', but of how copyright law defines derivative works. The following points of view exist:

Point of view: dynamic and static linking violate GPL

The Free Software Foundation (which holds the copyright of several notable GPL-licensed software products and of the license text itself) asserts that an executable that uses a dynamically linked library is indeed a derivative work. This does not, however, apply to separate programs communicating with one another.

The Free Software Foundation also created the LGPL, which is nearly identical to the GPL, but with additional permissions to allow linking for the purposes of "using the library".

Richard Stallman and the FSF specifically encourage library writers to license under the GPL so that proprietary programs cannot use the libraries, in an effort to protect the free software world by giving it more tools than the proprietary world.

Point of view: static linking violates GPL but unclear as of dynamic linking

Some people believe that while static linking produces derivative works, it is not clear whether an executable that dynamically links to a GPL code should be considered a derivative work (see weak copyleft). Linux author Linus Torvalds agrees that dynamic linking can create derived works but disagrees over the circumstances.

A Novell lawyer has written that dynamic linking not being derivative "makes sense" but is not "clear-cut", and that evidence for good-intentioned dynamic linking can be seen by the existence of proprietary Linux kernel drivers.

In '' Galoob v. Nintendo'', the United States Ninth Circuit Court of Appeals defined a derivative work as having form' or permanence" and noted that "the infringing work must incorporate a portion of the copyrighted work in some form", but there have been no clear court decisions to resolve this particular conflict.

Point of view: linking is irrelevant

According to an article in the ''Linux Journal'', Lawrence Rosen (a one-time Open Source Initiative general counsel) argues that the method of linking is mostly irrelevant to the question about whether a piece of software is a derivative work; more important is the question about whether the software was intended to interface with client software or libraries.Lawrence Rosen,
Derivative Works
, ''Linux Journal'' (1 January 2003).
He states, "The primary indication of whether a new program is a derivative work is whether the source code of the original program was used n a copy-paste sense modified, translated or otherwise changed in any way to create the new program. If not, then I would argue that it is not a derivative work," and lists numerous other points regarding intent, bundling, and linkage mechanism.
He further argues on his firm's website that such "market-based" factors are more important than the linking technique.

There is also the specific issue of whether a plugin or module (such as the NVidia or ATI graphics card kernel modules) must also be GPL if it could reasonably be considered its own work. This point of view suggests that reasonably separate plugins, or plugins for software designed to use plugins, could be licensed under an arbitrary license if the work is GPLv2. Of particular interest is the GPLv2 paragraph:

The GPLv3 has a different clause:

As a case study, some supposedly proprietary plugins and themes/ skins for GPLv2 CMS software such as Drupal and WordPress have come under fire, with both sides of the argument taken.

The FSF differentiates on how the plugin is being invoked. If the plugin is invoked through dynamic linkage and it performs function calls to the GPL program then it is most likely a derivative work.

Communicating and bundling with non-GPL programs

The mere act of communicating with other programs does not, by itself, require all software to be GPL; nor does distributing GPL software with non-GPL software. However, minor conditions must be followed that ensure the rights of GPL software are not restricted. The following is a quote from the gnu.org GPL FAQ, which describes to what extent software is allowed to communicate with and be bundled with GPL programs:

The FSF thus draws the line between "library" and "other program" via 1) "complexity" and "intimacy" of information exchange and 2) mechanism (rather than semantics), but resigns that the question is not clear-cut and that in complex situations, case law will decide.

Legal status

The first known violation of the GPL was in 1989, when NeXT extended the GCC compiler to support Objective-C, but did not publicly release the changes. After an inquiry they created a public patch. There was no lawsuit filed for this violation.

In 2002, MySQL AB sued Progress NuSphere for copyright and trademark infringement in US federal court. NuSphere had allegedly violated MySQL's copyright by linking MySQL's GPL-licensed code with NuSphere Gemini table without complying with the license. After a preliminary hearing on 27 February 2002, the parties entered settlement talks and eventually settled. After the hearing, FSF commented that the judge "made clear that she sees the GNU GPL to be an enforceable and binding license."

In August 2003, the SCO Group stated that they believed the GPL to have no legal validity and that they intended to pursue lawsuits over sections of code supposedly copied from SCO Unix into the Linux kernel. This was a problematic stand for them, as they had distributed Linux and other GPL-licensed code in their Caldera OpenLinux distribution, and there is little evidence that they had any legal right to do so except under the terms of the GPL. In February 2018, after a federal circuit court judgment, appeal, and the case being (partially) remanded to the circuit court, the parties restated their remaining claims and provided a plan to move toward final judgement. The remaining claims revolved around Project Monterey and were finally settled in November 2021 by IBM paying $14.25 million to the TSG (previously SCO) bankruptcy trustee.

In April 2004, the netfilter/iptables project was granted a preliminary injunction against Sitecom Germany by Munich District Court after Sitecom refused to desist from distributing Netfilter's GPL-licensed software in violation of the terms of the GPL. Harald Welte of Netfilter was represented by ifrOSS co-founder Till Jaeger. In July 2004, the German court confirmed this injunction as a final ruling against Sitecom. The court's justification was that:

:Defendant has infringed on the copyright of the plaintiff by offering the software 'netfilter/iptables' for download and by advertising its distribution, without adhering to the license conditions of the GPL. Said actions would only be permissible if the defendant had a license grant.... This is independent of the questions whether the licensing conditions of the GPL have been effectively agreed upon between plaintiff and defendant or not. If the GPL were not agreed upon by the parties, defendant would notwithstanding lack the necessary rights to copy, distribute, and make the software 'netfilter/iptables' publicly available.

This exactly mirrored the predictions given previously by the FSF's Eben Moglen. This ruling was important because it was the first time that a court had confirmed that violating terms of the GPL could be a copyright violation and established jurisprudence as to the enforceability of the GPLv2 under German law.

In May 2005, Daniel Wallace filed suit against the Free Software Foundation in the Southern District of Indiana, contending that the GPL is an illegal attempt to fix prices (at zero). The suit was dismissed in March 2006, on the grounds that Wallace had failed to state a valid antitrust claim; the court noted that "the GPL encourages, rather than discourages, free competition and the distribution of computer operating systems, the benefits of which directly pass to consumers".Dismissal
of Wallace v. FSF. Fro
this article
on Groklaw. Wallace was denied the possibility of further amending his complaint, and was ordered to pay the FSF's legal expenses.

On 8 September 2005, the Seoul Central District Court ruled that the GPL was not material to a case dealing with trade secrets derived from GPL-licensed work. Defendants argued that since it is impossible to maintain trade secrets while being compliant with GPL and distributing the work, they are not in breach of trade secrets. This argument was considered without ground.

On 6 September 2006, the gpl-violations.org project prevailed in court litigation against D-Link Germany GmbH regarding D-Link's copyright-infringing use of parts of the Linux kernel in storage devices they distributed. The judgment stated that the GPL is valid, legally binding, and stands in a German court.

In late 2007, BusyBox developers and the Software Freedom Law Center embarked upon a program to gain GPL compliance from distributors of BusyBox in embedded systems, suing those who would not comply. These were claimed to be the first US uses of courts for enforcement of GPL obligations. (See BusyBox GPL lawsuits.)

On 11 December 2008, the Free Software Foundation sued Cisco Systems, Inc. for copyright violations by its Linksys division, of the FSF's GPL-licensed coreutils, readline, Parted, Wget, GNU Compiler Collection, binutils, and GNU Debugger software packages, which Linksys distributes in the Linux firmware of its WRT54G wireless routers, as well as numerous other devices including DSL and Cable modems, Network Attached Storage devices, Voice-Over-IP gateways, virtual private network devices, and a home theater/media player device.

After six years of repeated complaints to Cisco by the FSF, claims by Cisco that they would correct, or were correcting, their compliance problems (not providing complete copies of all source code and their modifications), of repeated new violations being discovered and reported with more products, and lack of action by Linksys (a process described on the FSF blog as a "five-years-running game of Whack-a-Mole") the FSF took them to court.

Cisco settled the case six months later by agreeing "to appoint a Free Software Director for Linksys" to ensure compliance, "to notify previous recipients of Linksys products containing FSF programs of their rights under the GPL," to make source code of FSF programs freely available on its website, and to make a monetary contribution to the FSF.

In 2011, it was noticed that GNU Emacs had been accidentally releasing some binaries without corresponding source code for two years, contrary to the intended spirit of the GPL, resulting in a copyright violation. Richard Stallman described this incident as a "very bad mistake", which was promptly fixed. The FSF did not sue any downstream redistributors who also unknowingly violated the GPL by distributing these binaries.

In 2017 Artifex, the maker of Ghostscript, sued Hancom, the maker of an office suite that included Ghostscript. Artifex offers two licenses for Ghostscript; one is the AGPL License and the other is a commercial license. Hancom did not acquire a commercial license from Artifex nor did it release its office suite as free software. Artifex sued Hancom in US District Court and made two claims. First, Hancom's use of Ghostscript was a violation of copyright; and second, Hancom's use of Ghostscript was a license violation. The court found the GPL license was an enforceable contract and Hancom was in breach of contract.

On 20 July 2021, the developers of the open-source Stockfish chess engine sued ChessBase, a creator of chess software, for violating the GPLv3 license. It was claimed that Chessbase had made only slight modifications to the Stockfish code and sold the new engines (Fat Fritz 2 and Houdini 6) to their customers. Additionally, Fat Fritz 2 was marketed as if it was an innovative engine. ChessBase had infringed on the license by not distributing these products as Free Software in accordance with the GPL.

A year later on 7 November 2022, the parties reached an agreement and ended the dispute. In the near future ChessBase will no longer sell products containing Stockfish code, while informing their customers of this fact with an appropriate notice on their web pages. However, one year later, Chessbase's license would be reinstated. Stockfish did not seek damages or financial compensation.

Compatibility and multi-licensing

Code licensed under several other licenses can be combined with a program under the GPL without conflict, as long as the combination of restrictions on the work as a whole does not put any additional restrictions beyond what GPL allows. In addition to the regular terms of the GPL, there are additional restrictions and permissions one can apply:

# If a user wants to combine code licensed under different versions of GPL, then this is only allowed if the code with the earlier GPL version includes an "or any later version" statement. For instance, the GPLv3-licensed GNU LibreDWG library cannot be used by LibreCAD and FreeCAD who have GPLv2-only dependencies.
# Code licensed under LGPL is permitted to be linked with any other code no matter what license that code has, though the LGPL does add additional requirements for the combined work. LGPLv3 and GPLv2-only can thus commonly not be linked, as the combined Code work would add additional LGPLv3 requirements on top of the GPLv2-only licensed software. Code licensed under LGPLv2.x without the "any later version" statement can be relicensed if the whole combined work is licensed to GPLv2 or GPLv3.

FSF maintains a list of GPL- compatible free software licenses containing many of the most common free software licenses, such as the original MIT/X license, the BSD license (in its current 3-clause form), and the Artistic License 2.0.

Starting from GPLv3, it is unilaterally compatible for materials (like text and other media) under Creative Commons Attribution-ShareAlike 4.0 International License to be remixed into the GPL-licensed materials (prominently software), not vice versa, for niche use cases like game engine (GPL) with game scripts (CC BY-SA).

David A. Wheeler has advocated that free/open source software developers use only GPL-compatible licenses, because doing otherwise makes it difficult for others to participate and contribute code. As a specific example of license incompatibility, Sun Microsystems' ZFS cannot be included in the GPL-licensed Linux kernel, because it is licensed under the GPL-incompatible Common Development and Distribution License. Furthermore, ZFS is protected by patents, so distributing an independently developed GPL-ed implementation would still require Oracle's permission.

A number of businesses use multi-licensing to distribute a GPL version and sell a proprietary