The GtkRadiant sources as originally released under the GPL license.

README.txt

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+Fri Feb 17 11:10:50 CST 2006
+
+If you are getting this file long after our initial release,
+chances are the source is quite outdated. See the release
+announcement below.
+
+TTimo
+
+id Software releases GtkRadiant source code under GPL
+=====================================================
+
+Following last summer's release of Quake III Arena source code and tools
+under the GPL license, id is now placing GtkRadiant and q3map2 under GPL
+license. We are also providing in this release a number of Quake II
+tools that never made it under GPL when we packaged Quake II source code.
+
+GtkRadiant [1] is the heavily modified version of QERadiant and
+Q3Radiant ( id's level editors used during Quake II and Quake III Arena
+development ). It runs on Microsoft Windows platforms, GNU/Linux, MacOS
+X, and supports level editing for many id technology games [2].
+
+A tarball of the initial release is available from our ftp server [3],
+and the development website [4] now holds a subversion repository with
+the GPL code. We expect development on the editor to continue as usual,
+and hope that many new developers will join us [5] :-)
+
+[1] http://www.qeradiant.com/
+[2] http://www.qeradiant.com/?data=sitemap#games
+[3] ftp://ftp.idsoftware.com/idstuff/source/GtkRadiant-GPL.zip
+[4] http://zerowing.idsoftware.com/
+[5] http://www.qeradiant.com/wikifaq/index.php?How%20to%20reach%20the%20developers