AFAIU, the GUS was basically a DSP + sample RAM. With an appropriate driver, it should have been possible to load General MIDI instruments (maybe only what's needed for the game you are about to play, due to limited capacity) and let the DSP handle pitch and effects. It seems like such drivers existed, called ULTRAMID and MEGAEM. Whether that was a common thing to do at the time - no idea.

But what the GUS was popular for was games (and demoscene demos!) that used the hardware "natively" by loading their own samples and maybe own effects.