Do you want to play GB and GBC
GB Bridge is an adapter between GBA Flash Advance Pro or Fash2Advance
cartridge and GBA. With it you will be able to play GameBoy and
GameBoy COLOR roms on your GBA and use Flash Advance card to play
GB and GBC roms on Gameboy, GB Pocket and GB Color. Compatible
with Flash Advance and Flash Xtreme and Flash2Advance Cards.
You need this adapter becouse old Gameboy and GB Color cartridges
were made for 5.5V (used 4 batteries), but now the new cartridges
have internal Voltage of 3V (2 batteries), so for your Gameboy
to recognise the Flash Cartridge as the old GB card and let you
play .GB and GBC roms you need an adapter. But Play original free
gba roms without it.
GBA ROMS FAQ