The iPhone 3GS wasn’t Apple’s first “GS” computer. Back in 1986, Apple released the Apple IIGS, where the GS stood for Graphics and Sound.
The iPhone 3GS’s S stands for Speed, and the G comes from “Generation”, as in “3rd generation wireless data”.
I found two sites that compared the IIGS with the 3GS:
Learning in Hand – side-by-side comparison table. Interesting to note that the IIGS had over 10,000 apps, but it took 17 years for those apps to be developed. Programming tools and education (among other things) have advanced so much that the iPhone now has 140,000 apps, and it took less than 3 years to get there.
iPhone J.D. – thoughts on the 3GS. Notably, the iPhone is far faster than the IIgs ever was :)