mGB is a Game Boy program that plays MIDI notes received through the Game Boy's link port, turning the Game Boy into a MIDI synthesizer.
mGB can be loaded onto a nanoloop one cart as extra ROM. Currently, patch saving is not supported on nanoloop one. Support for nanoloop's save format is under development.
mGB was originally developed for Arduinoboy, a device that converts data from a MIDI-cable to the format of the Game Boy's link port. The nanoloop USB-MIDI adaptor does the same for USB-MIDI (today, most computers don't have a real MIDI-port but use virtual MIDI over USB) and works with mGB, too.
More information on Arduinoboy and mGB can be found here:
Arduinoboy / mGB homepage