You can get plenty of Apple IIGS games here:
http://www.whatisthe2gs.apple2.org.za/the_fairway/index.html
Including ICOM adventures that originated on the Mac (but in colour) and the Neuromancer adventure game. The Immortal is supposed to be good. Haven't played it myself.
Yeah, that site is pretty old, I think I've leeched everything off of it by now.

I really like the site design over there but they have a fairly limited selection of //GS stuff.
I think you asked me in another thread what kind of development was happening for the //GS these days. The answer is.... everything! There is quite a bit happening on the hardware end. Go to this site:
www.gse-reactive.com and go into the "Store" to see a bunch of recently developed hardware items. IDE controllers, compact flash drives, sound cards, memory, 1.4MB floppy controllers & drives, etc. There has been a brand-new Ethernet card produced by another party. On the software side, there are still games, OS extensions, utilities, you name it being created. Particularly interesting to me is a TCP/IP stack extension for the OS, combined with a new web serving application that allows you to use your //GS as a real, live webserver.
Back on topic, BTW: I downloaded and played Ys I on my //GS last night. It's a pretty decent port! Obviously the graphics aren't as nice as the TurboGrafx version, which in turn aren't as nice as they could be, but it's really solid. And it actually has a "Quit" function from within the game, unlike a lot of the seriously dated //GS games that require you to reboot to quit. How annoying is that?
EDIT: Here's one item in particular I'm looking at for my //GS:
http://www.gse-reactive.com/product_info.php?products_id=126. It's a little pricey but I think it'd be worth it in the long run.