The source was released under the GNU General Public License.
The engine is available under the GNU General Public License.
It is released under the terms of the GNU General Public License.
In 1989, the first version of the GNU General Public License was published.
It is available free of charge under the GNU General Public License.
This software is free under GNU General Public License 2.0.
The program is released under the GNU General Public License.
The game is free software under the GNU General Public License.
It is available as free software under the GNU General Public License.
Released under the terms of GNU General Public License 3.