It works in 2 ways..You can drive the harmoniser from data contained in a MIDI track, or you can have it produce live harmonies based on the audio input from your mic. It also has a footswitch that can be used to bypass the harmony/effects or not.
The difference between the G and the M is that the G cannot be used by sending it midi data it only uses the live audio data coming from a mic.
It will be a great unit if you only want/need 2 harmony voices. It continues with the great helicon effects though.