The Commodore 64 Games System (C64GS) was the cartridge-based game console version of the popular Commodore 64 home computer.
When you look at the C64GS, the size of the case already gives a hint at what’s inside: basically a standard C64 board.
The userport and cassette port are still there (but not exposed to the outside as these were not in use on the GS systems), and there’s even the placeholder for the keyboard connector (which is obsolete as well as the GS is a pure console).
The only notable difference is the ROM chip, which contains two important modification to a standard C64 ROM. The first is that switching on the machine without a cartridge present results in an animation asking the user to insert a cartridge and showing the correct power-on sequence. The second is an additional set of windowing commands, designed to compensate for the lack of a keyboard. There is however no known software that uses it.
With the C64GS being basically a cut-down version of a regular C64, it never really caught on and only a small amount of them were made (reports indicate anything between 5000 and 10000 units). The traditional C64 could handle basically everything the C64GS could do and more.
Next to that, the release of the C64GS in a time where companies like Sega and Nintendo were releasing their 16-bit consoles (i.e. the Mega Drive and the Super NES) meant that it had to deal with some very tough competition right from day one.
So, although not Commodore’s biggest commercial success, it still is one of the most sought after pieces of Commodore hardware today, partly because of the fact that it is quite rare and partly because it was the last of the C64 line of products to come out of the factories that brought us the best selling computer of all time.