David DeGraw of Catskull Electronics has developed Teensyboy, an awesome little MIDI adapter for a Nintendo Gameboy.
This compact little shield is about the size of thumb drive and uses USB MIDI. Simply plug it into into a computer and fire up your favorite digital audio workstation (DAW) and you’ll be jamming with your Gameboy before you know it.
There is also a Teensyboy Pro that includes a variety of modes to accommodate Little Sound DJ (LSDJ), Nanoloop and mGB. It also has convenient mounting holes for installation into a case.
Vague Robots has some tutorial videos on using the Teensyboy Pro with LSDJ