M1 GPU is great. But only when you compare it to iGPU on Intel macs. Even AMD 5300M is waaay better than M1 GPU. I highly doubt Apple can compete with AMD/Nvidia on GPU. But than again, I've thought Apple couldn't compete with Intel in their first gen CPUs, so it's not like I have a great track record when it comes to predicting stuff
Apple already kicked back Imagination on A-series mobile SoCs and started rolling their own GPUs several years ago.
My guess is that they already had something comparable to a mid-range AMD discrete GPU in the lab for a couple of years and they are refining that design.
Remember that the M1 wasn't "just as good" as the Intel CPU it replaced nor was it slightly better. It was a
big step forward and it is likely they won't release their own discrete graphics hardware until they can do the same.
This all goes back to the A7 (2013), the first 64-bit mobile processor with wide mainstream distribution. The rest of the semiconductor industry was shocked speechless.