Well the most well known one is Geos for the C64

https://youtu.be/_4nthOx8sA4?si=AiK9bRxRQwV3MB0f

There's also this Atari homebrew

https://youtu.be/T14dL9MeMHE?si=cGtsZGWILYi4jcql

And yes the IIGS had one

https://youtu.be/YvVFTpukAp0?si=UtvV3N_pKhxU-Tyb

One comment on the IIgs vs the C64; the IIgs had a 16-bit CPU and tons of RAM, so GS/OS is not necessarily representative of what is possible on a 6502.