The original Quake 2 sources as originally released under the GPL license on December 21, 2001.

readme.txt

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+
+This is the complete source code for Quake 2, version 3.19, buildable with
+visual C++ 6.0.  The linux version should be buildable, but we haven't
+tested it for the release.
+
+The code is all licensed under the terms of the GPL (gnu public license).  
+You should read the entire license, but the gist of it is that you can do 
+anything you want with the code, including sell your new version.  The catch 
+is that if you distribute new binary versions, you are required to make the 
+entire source code available for free to everyone.
+
+The primary intent of this release is for entertainment and educational 
+purposes, but the GPL does allow commercial exploitation if you obey the 
+full license.  If you want to do something commercial and you just can't bear 
+to have your source changes released, we could still negotiate a separate 
+license agreement (for $$$), but I would encourage you to just live with the 
+GPL.
+
+All of the Q2 data files remain copyrighted and licensed under the 
+original terms, so you cannot redistribute data from the original game, but if 
+you do a true total conversion, you can create a standalone game based on 
+this code.
+
+Thanks to Robert Duffy for doing the grunt work of building this release.
+
+John Carmack
+Id Software
+
+