I like 8-bit Apples, but I'm not a huge fan of GS/OS. Here's a few reasons why:
* A GUI with those funky rectangular pixels does't quite work for me. The Amiga sorta gets away with it because it was used for video work and had a shitload of games, but the IIgs barely qualifies in either category.
* The colour depth on the IIgs is too limited. The Mac got it right by going black-and-white with more pixels makes it much more useful for both applications and games. (Think Return to Dark Castle for a wonderful black-and-white game.) It is possible to sneak more colour onto the screen, but I've never seen it applied to animations on the IIgs.
* The system is too bloody slow, at least for GUI applications given the design of GS/OS. The IIgs is brilliant for 8-bit applications because 8-bit applications were designed for a 1 MHz processor. Alas, it feels like most GS/OS software was designed for a 7 MHz processor.
* As Unknown_K said, the sheer number of applications and games. System 6 will lose out to System 7 in that category, but System 6 is still much more impessive than GS/OS. You can realistically do everything from word processing to video editing with appropriate hardware with System 6. GS/OS will give you half-way to WYSIWYG word processing, and video is limited to still captures or the overlay card.
I'm not sure if an Amiga or Atari ST of the same era will hold up nearly as well as System 6 when it comes down to application support, but they are still somewhat better than GS/OS. (They aren't much better than 8-bit Apple II software though. The old Apple II's did have more choice, but they also had more limitations.)
In a very real sense, the 8-bit Apple and the Macintosh were the death of ISV support for the IIgs.