My hardware design prof in undergrad had a story about one of the chip manufacturers having some cray fast internal clock in the 70s or 80s that was running microcode to implement the processor’s ISA. In my (probably faulty) memory I think the story was something like 800 MHz made by a weird brand (weird to my naive undergrad brain) like Rockwell or something, while the instruction clock presented to the user was maybe in the single or double digit MHz. No idea if I’m remembering right, and my Google Fu is failing to verify this story. But anyway, to your point, modern GPUs have a bunch of different clocks for different sub-systems.