The 64GS was one of Commodore’s last gasps at trying to make some money using the 8-bit parts they still had left in stock. The whole thing was a disaster.
It wasn’t based on the C64. It was a C64. Without a keyboard and some of the other ports missing. A fact that came to bite anyone who tried a C64 cartridge game that needed keyboard input.
And IIRC one of the games that came bundled with it was a game like that.
They were at least smart enough to have the BASIC startup pointer (the one that otherwise caused READY. to appear) in the ROM patched to go to a neat little graphic telling people to turn it off, plug in a game and turn it back on again.
What Commodore saved by releasing the GS, the customer ultimately paid by needing to buy games in a format more expensive than disk or tape that would run on a regular C64.
… and given the time period, lots of people were buying PCs and offloading their regular C64 hardware and a ton of games for the price of the GS and its handful of games. And that C64 would run any GS game that was likely to come out.
Terry Davis tried to do for the PC with TempleOS what the C64’s BASIC and KERNAL did for its hardware.
Terry was all the more a mad lad because he didn’t get to create the hardware spec he was working with.
Could you imagine someone doing the same as Commodore did but starting with 64-bit era hardware?
Taking it another direction, there are free and paid “easy programming” platforms that provide a sandbox not unlike a modern version of what it was like to program a C64.
At a pinch, DOSBox and a copy of QBASIC might suffice.