Do you want to play GB and GBC
GB Bridge is an adapter between GBA Flash Advance 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.