Spout is probably the most addictive GP32 game.
It runs with 33(66?) Mhz (can't remember, but I know it was the lowest the gp32 could go - so you should have plenty of time left.
http://www.gp32x.de/cgi-bin/cfiles.cgi?0,0,0,0,30,314
The image doesn't look so cool, but the gameplay is...