@Mark, Z6 users have a GB/GBC/GBA combo emulator but the GBA Games speed is too slow to be considered playable!
You, on the other hand, have a good GBA emulation speed and working sound.
The only SDL based emulator that loads GB/GBC roms is good old VBA, though it's not the best GBC emulator by emulation (I can assure you that because I have a real GBC)
Don't know about you, but to my mind, the Game Boy Color had a lot more brilliant games to play than GBA, so I can guess why you ask for a GB emulator :)

@Pantamorph
if you have gba emulator but the ROKR E2 is not compatible with gb and gbc games:)

@Mark
Isn't there already a GBA emulator for E2? o_O

excellent works only need a emulator game boy and game boy color xD

slow fps :(

very good the emulator, could not be changed by using the keys Joystick 4 6 2 8 because the Joystick is very fragile.

no problem, it can easy reconfigure use menu
Machine setting -> joystick setting -> Define keysets