In 1992, there was a lot of hype and excitement built up about the GUS - especially on USENET. I eagerly bought a GUS immediately when it became available in November, 1992 (spending $US 160 on it). It replaced an original Soundblaster v1.5 in my 386/33 system.
The difference in sound quality was amazing. For the time, the GUS was incredible - when supported natively. Games, MOD trackers, demos, MIDI tracks all sounded great with native GUS support. For everything else, you had to use a TSR for Sound Blaster emulation ("SBOS") or MT-32 emulation ("MEGAEM" - I think? ). Both worked poorly - if at all - with games and took up precious memory. Newer software versions released over time worked a little better but took up even more memory.
As a college student at the time, I had to sell my SB to help fund the GUS purchase, so having both cards in my system wasn't an option. I don't think that I knew that having two sound cards was even possible.
When I built a 486DX2/80 system in early 1995, that original GUS moved into it from the 386. Newer/better versions of the GUS came out over time - including the latter versions that had SB support in hardware, but I kept using my "classic" GUS.
When I finally built a Pentium 200MMX system in late 1997, I attempted to install my classic GUS in it, but I simply couldn't get it to work right in Win95. After five years of use, I had to give up on the GUS, and I replaced it with a SB AWE64 Gold.
I don't recall exactly what happened to my original GUS. It probably went with my old 486 when I gave it away.
Like the OP, I lucked into a free classic GUS a few years ago. I also have a PicoGUS but haven't had time to mess with it.