The original Doom source code was released by id Software under the
GPL v2, but has been relicensed under the GPL v3 for GZDoom.
See gpl.txt

Parts of the voxel code in the software renderer use code from the 
BUILD engine by Ken Silverman and are used under the terms of the
GPL v3 with permission.

The majority of original code uses a BSD-like lincese. See bsd.txt.

The OpenGL renderer is released under the LGPL v3, except some bits
of code that were inherited fro ZDoomGL.

This software is based in part on the work of the Independent JPEG Group.

This software uses the 'zlib' general purpose compression library by
Jean-loup Gailly and Mark Adler.

This software uses the gdtoa package, see gdtoa.txt.

This software uses the game_music_emu library, which is covered by the GNU Lesser
General Public License. See lgpl.txt.

This software uses the "Dynamic Universal Music Bibliotheque" library for
MOD music playback. See dumb.txt for original license. The version used,
however, has been heavily modified from its original form and is the same
version used by the foobar2000 component foo_dumb as of mid-2008, found at
http://kode54.foobar2000.org/.

All script code in gzdoom.pk3 is licensed under the GPL v3 unless noted 
otherwise.