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.
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.