GBS is a video game music format much like the NSF/SID file formats that extracts the music from a ROM that came from the actuall cartridge that someone managed to dump and transfer to a PC. In this case, GBS's are from Gameboy/Color game ROM's. And some players have been written to play these music files.

You can play these GBS music files with Winamp 2.91. You can get the video game music plugins at Zophar's Domain.