There is quite a large number of games for GnuLinux, contrary to common belief. In a shop for computer games in Denmark on the 20th of Marts 2004, it turned out the shop sold a game called UnrealTournament2004 which apparently also worked under GnuLinux.
There are also games for GnuLinux that doesn't run on many other platforms, like Pingus and Armagetron.
Of late, there even exists a online forum on efnet IRC called #LinuxGames, which has focus on FreeAsInBeer games for GnuLinux. The channel has a homepage http://linuxgames.no-ip.info/linux/
There is also a simular channel ( #GameTome? ) on freenode (irc.gnu.org), they also have a homepage: http://www.happypenguin.org/
NOTE: Chances are your distribution of GnuLinux will contain a large number of games allready. DebianGnuLinux has a nice collection of games for instance. So, if you want, say, FrozenBubble? for DebianGnuLinux you can just AptGet it.
Examples:
apt-get install liquidwar apt-get install bomberclone apt-get install armagetronGames and related: