The Commodore C64GS (Commodore 64 Game System) is basically a C64 motherboard, in a simple beige plastic box without a keyboard, serial port and cassette player contacts, using the C64 modified to get the cartridge on top.
Aside from minimal and minor changes to the ROM, the circuitboard is just a C64. So it can play all C64 cartridges that are not dependent on the keyboard.
Some cartridges were made specifically for the C64 GS, but were very few. These cassettes could also be used on the C64 or C128 that had cartridges.
It was a Commodore failure that tried with the C64GS to compete with the Nintendo Entertainment System and Sega Master System.
The reasons for the failure were the poor support of game development companies, the existence of the C64 itself and outdated technology. Let's not forget that the competing consoles were 16-bit technology when Commodore C64GS is 8-bit.