Interesting write up about the C64 gs and as expected, the only thing they did was rotate the cartridge port (ok and change a OS chip I guess).
The article even says that unsold C64 gs-es were brought back to Commodore and turned into regular C64's !!!
Now that's a story that IMHO is even more interesting than the "famous" Atari E.T. dump, but you never hear about such stuff strangely
http://www.infinitefrontiers.0catch.com/CommodoreC64History.htmInside pic:

Very nice collectors item !
The system failed for the same reasons the XEGS didn't do very well: old hardware compared to the existing computers (Amiga, ST) and game consoles already on the market. The XEGS had the luck of a really, really big back-catalog of great games AND (surprisingly) Atari actually release a good number of new cartridges for it. Some were totally new games, some were games that used to be (big) floppy-disk only games but now were bank-switched and pushed into a cartridge (normally an A8 cartridge max. size is 16K but they by-passed this limit with some tricks).
Two of these were Ballblazer and Rescue on Fractalus.
IMHO another big mistake for the C64 gs was making it a 100% cartridge system only, you couldn't turn it into a full C64, no separate keyboard, no ports to connect floppy drives/printers etc.