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Basically the license give users the right to view, learn from, use, share, and modify the source code, under the condition that the same license apply to all modifications of the source code. This makes sure that free software remain free (“free as in speech”). The license does NOT forbid to make money out of a program, it demands only that the source code must be made public.
You can find the GPL license here:
For game developers, using the gpl license for the source code of a game means that this source code must be made public and can be used, viewed and edited by other people. It is still possible to generate money out of a gpl-licensed game:
from the website: http://www.gnu.org/copyleft/gpl.html
<one line to give the program's name and a brief idea of what it does.> Copyright (C) <year> <name of author>
This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>.
<program> Copyright (C) <year> <name of author> This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'. This is free software, and you are welcome to redistribute it under certain conditions; type `show c' for details.