Remember the GPL only applies to the code you can make a great game with beautiful artwork and distribute the source code to anyone who wants it. Nobody playing the game will have much fun without the artwork.
Carmack has a post from ages ago wondering why no one does that with the ID engines they open sourced, which were pretty current back then. He was talking about the quake (2?) source code dumps i think.
The GPL license will allow people to take the Quake 3 engine and even go so far as to release a commercial product with it - provided that the source code is published alongside. Nobody has done this with any of the Quake engine games yet, but he hopes to see it happen someday.
Edit: ohh i found it:
http://www.gamespy.com/articles/641/641662p6.html
The GPL license will allow people to take the Quake 3 engine and even go so far as to release a commercial product with it - provided that the source code is published alongside. Nobody has done this with any of the Quake engine games yet, but he hopes to see it happen someday.