Just want to set the record straight on a couple of things:

The FSF considers art to be non-functional data that does not trigger the GPL's linking requirement:

Data that has an aesthetic purpose, rather than a functional one, may be included in a free system distribution as long as its license gives you permission to copy and redistribute, both for commercial and non-commercial purposes. For example, there are some game engines that have been released under the GNU GPL, and have accompanying game information—a world map, game graphics, and so on—released under such a verbatim distribution license. This kind of data can be part of a free system distribution.

Source: http://www.gnu.org/philosophy/free-system-distribution-guidelines.html

Secondly, the art will be dual licensed as CC-BY-SA, so even if you happen to not like the GPL, you need not use the art under that license. Also, while the contest requires your art to be CC-BY-SA and GPL for the sake of consistency, you are also entirely free to license it under a less restrictive license as well.

Finally, your assertion that the game has to run only on a 100% free platform is false. Your game can run on any platforms you want it to. It's just that those platforms must include a 100% free platform.

We don't want to restrict what you do with your code and art. However, we do have a set of standards for entry -- as long as those standards are met, you can do anything you want with your work.