I find GPSP annoying to configure (BIOS has to go in emulator’s directory, button mapping is different). GPSP was traditionally used instead of Retroarch because of performance, but I suspect this will be different on the Pi2.
I propose adding RetroArch GBA to the main image (but keep GPSP and let users choose between them as has been done with other emulators).
If anyone knows how to install Retroarch-GBA manually, please let me know so I can test it :)