C, C++, Java, and Python are your main options for programming on the GCW Zero for now. Python will be the slowest. There are currently game creation engines like RPG Maker etc being ported and the effort will surely continue. You can tack Perl onto the list if you like but I'm not sure anything Perl has ever been ported. It surely works. I am not 100% sure of the viability of Java right *now* but it worked great on GP2X which is a similar but much lower-power device so I cannot see it not working on GCW Zero.
I had a good go at compiling Mono for GCW but the uclibc of GCW and infantile MIPS support of Mono got in the way. I had little success as of March 2013, the last time I tried.