I'm pretty sure a GUS still in its box would turn up a rather good price from nostalgia enthusiasts.
I mostly remember it for the fact that a great many demoscene demos and even some games would only play sound (music) if you had a GUS, a Soundblaster wasn't enough.
I think it had something to do with it being easier to play MODs on a GUS, or something. I imagine this was because you could load sounds into its memory and trigger them? I'm really not sure, someone please clarify :)
Either way, make sure you get a good price for that GUS :) Or better, make sure it finds a home with a loving enthusiast :)