A game console released by Commodore; Introduced in Europe in 1989-1991.
Commodore 64GS had a "new" (BN/E) Commodore 64 motherboard. The case was smaller than C64 case; There was no keyboard, no holes for different ports; the only ports were for the video, joysticks and the cartridge hole that was on top of the unit. The Kernal ROM was modified so that it didn't boot to BASIC if you turned the machine on with no cartridge; instead it just displayed a helpful animation telling the user how to insert the cartridge. (Apparently the machine had a BASIC ROM and even some advanced windowing things in the ROM, who knows what for...)
So, basically it was a severely crippled Commodore 64.
Why did Commodore do this? They figured that C64 as a hardware platform had definitely proven its worth. Besides, while C64 cartridges originally contained only 32K RAM, the newest technological advanced allowed the cartridge manufacturers to make whoppin' 4 megabit cartridges (that is, 512 kilobytes; though, I saw it being advertised as 2 or 4 megabytes!). Cartridges were seen as the Savior; Cartridges had none of the slowness of floppies or tape, and they were Easy to Use. (Not by people like me who had fragile-sounding disk turbo modules plugged in for most of the time, though... =)
This belief was demonstrated to be true later with the release of System 3's Last Ninja III - the gigantic game reportedly worked much better on a cartridge.
Regrettably, everyone who was into C64 stuff already used C64, and the real golden age of C64 was coming to end by early 1990s - and newer and better machines were coming fast. It was a really massive flop - 80000 machines produced, approximately 25% of them sold. The remainder were apparently recycled into C64Gs.
See also UltiMax. Not the same thing as Commodore 64G, thank Heavens!
Source: http://www.floodgap.com/retrobits/ckb/secret/ultimax.html
http://mess.emuverse.com/sysinfo/c64.htm
and my memories from the Commodore mags of the era =)