A vaguely similar thread on AW led me to
flightgear - a GPL flight sim of incredible magnificance! I'm busily trying to get my OpenGL to work again (X doesn't like my KT400 chipset...).
There's over 9GiB of satellite photos for the terrain alone!
THIS game looks so awesome, can't wait to play it (X takes years to compile)!
This has to be the BIGGEST GPL'd game I've seen, as far as "artwork" goes but I guess in this case the real world is pretty big to pull "artwork" from ;-)
IMHO that's the problem with a lot of GPL games if you try to compare them to commercal stuff... OK game engines, but they lack the manpower for decent artwork in any reasonable volume.
- Paul
[edit: I would like to think that if I attempted to port any game, this is what I would try "First" ;-) ]