There is an open source driver for Vivante GPUs, including the GC400 (or maybe GCNano? and the official docs are oddly vague on what GPU is used and 3rd party sources disagree) used in the STM32MP157: etnaviv. I can't seem to find anything on what's in the STM32MP2 though other than that it supports Vulkan though. Not sure why ST is so tight-lipped on what GPU IP they're using