But if the engine is proprietary, how can a game made with that engine be
GPL?
It's because while the Duke Nukem 3-D source code is under the GPL, the BUILD
Engine is under a proprietary license. It's the same sort of situation as a
game under the GPL that uses Unity3d.
Kek, you can only distribute through the internet (so vague, FTP? WWW?
Torrents?)!
Eduke32 is released under the GPLv2 and the BUILD license, but the two seem
to conflict with one another, since the later prohibits commercial
distribution. So I'm a little confused as to whether it's fully free or not.
http://svn.eduke32.com/eduke32/polymer/eduke32/build/buildlic.txt
