You know Id Software open sourced many of their engines[0] and that have spawned tons of derived works[1].
0. https://github.com/id-Software 1. http://en.whttp://en.wikipedia.org/wiki/File:Quake_-_family_...
0. https://github.com/id-Software 1. http://en.whttp://en.wikipedia.org/wiki/File:Quake_-_family_...
It's still a really cool thing to do, but neither Id (or any other game company I'm aware of, anywhere, ever), has released their game as GPL while they were still making significant amounts of money from it (which in practice means within the first couple of months, for almost all games).