Sorry if this has already been said, but I don't think that it's even technically possible to fully emulate the GBA on the GP32 (At least at a decent speed). The GP32 is having a bit of trouble emulating 16 bit machines (But currently doing an excellent job on the SNES.
Keep going with that OpenSnes!), and for a 32 bit machine to emulate another 32 bit machine would be a bit slow...