GUS is shorthand for
Gravis Ultrasound, the fattest
ISA soundcard available for PC. Released around 1993-1994, it featured a lot of snacks that made the
SoundBlaster seem like a 1st year ee student practice assignment.
The GUS Max was an enhanced version. Some of the features are 48khz, 16bit, stereo waveform support, 32 independent hardware-mixed channels with up to 1mb of wavetable RAM (a lot back then) (making the card ideal for MOD type music, requiring almost no CPU power at all for mixing) and almost no noise at all (compared to SB16).
The GUS was a big hit especially in demo scene circles because it was so easy to program and had hardware mixing. Its soundblaster and Roland MIDI emulation was rather substandard though, but most games supported the card natively.
Sadly, the only useable windows drivers available never got through the beta stage before Gravis decided to get out of the sound card business. Some claim that the reason for this was that while Creative kept on releasing SB16, SB32, SB64 etc. having people upgrade every year, the GUS was so superior that nobody ever needed to ugprade their sound cards ever again. :)
I still have that full-size 16bit ISA card somewhere. It looks really cool and stands out with its blazing red color compared to the dull green every other expansion card I've ever seen.
http://listen.to/gusemu/index.html has a GUS emulator for windows that might be worth checking out.