Gradius II is a bit unique in how it plays back music; you have 3 options: Internal, MIDI MT-32 or MIDI SC-55.
However, the MIDI MT-32 option is not as simple as it sounds; it actually plays part of the music back via the YM2151 and uses the MT-32 to fill-in and complement the sounds that the internal sound chip makes. If you mute the YM2151 output and just listen to the MT-32, it's clear that it's only playing part of the melody. It actually works really well; you get the crisp, sharp sounds from the YM2151 synth and the warmer, deeper sounds from the Roland unit.
In MIDI SC-55 mode the internal YM2151 plays no part in the music output - all sounds are generated by the Sound Canvas module.