Yeah... Demos and mod trackers made a lot of GUS use. From what I remember, the audio synthesis model was fairly similar. I bet there was a lot of overlap between mod tracker authors and GUS enthusiasts. (And from what I remember, the MOD format itself was strongly derived from the capabilities of the Amiga audio hardware.)
I never owned a GUS, but I seem to remember most games having support for it. It always seemed to taunt me from the game's installer.
Or maybe I'm just thinking of demos and modtrackers. A lot of them used to require a GUS for sound, I think?