License: GNU GPL 1 or 2

The GNU GPL is a non-propriety license (FLOSS or Free/Libre Open Source Software) that is authored by the Free Software Foundation and endorsed by the Open Source Initiative. GPL licensed software is copyrighted. But rather than restrict the rights of others, the GPL uses the legal force of copyright law to hinder efforts by anyone to restrict the rights to use, copy, redistribute, study, modify, improve, and/or redistribute copies, improvements, and/or modifications, for any purpose, including the right to charge for services provided when exercising these rights and the right to make a profit from such services.

Generally GPL is unsuitable for text, art and other non-source code media that is separate from the application. GNU FDL or other licenses are recommended for such instead (as in, application to them is vague in terms of legality). However, art, text, and files distributed within executables and source as a single unit are GPL by default as well. For them to use a different license that is not compatible with the GPL they must be distributed as a separate entity.

Games can begin life under the GPL license, or be switched to the GPL from a proprietary license or other another open source license. Please add the tag for the other license and license-changed in such cases. Some times, source code for commercial games under a proprietary license are published via the GPL while content remains non-free. Please indicate the proprietary nature of the content in those cases

GPL version 3 includes clauses that make it fundamentally different from versions 1 and 2 which may have important effects for gamers and is therefore a separate group

Public Domain code and GPL code can be mixed together in variety of ways. A copyright holder of GPL code can change their license to Public Domain. But someone else cannot change a code's license to Public Domain if they are not the copyright holder. ie: The Free Software Foundation is the copyright holder for GNU Chess (GPL licensed code). I can make a Deep GNU Chess game using their GPL code. Deep GNU Chess cannot be Public Domain because it uses code that The Free Software Foundation owns. Neither can I say that just the bits of new code that I created are Public Domain. Because my code modifies FSF's code, it therefore becomes their code under the terms of GPL. Note that clauses in GPL version 3 may actually allow GPL3 code to be made Public Domain by someone who does not own the copyright. If someone claims Public Domain for modified or used GPL1 or GPL2 code, and is not the owner of said code, license-contradictory applies.