GS/OS can run off a single 800K floppy, if you're just curious about it... if you actually want to use it for anything you'll probably want more RAM and some kind of mass storage.

GS/OS 5.x is available if you look in the right places, but it's not good for much IMHO.

Incidentally, the original GUI that shipped with the IIgs was Apple II Desktop (AKA Mouse Desk), which is actually an 8-bit desktop.

Personally I've always found GS/OS to be a really slow, ugly and annoying copy of the Mac OS... you'll be miles ahead using ProSel 16.