Hi!
Well, GPU is a difficult situation right now. There are the closed source drivers which have a very incomplete state and need a highly patched Mesa to work. And even then, wayland is much better supported than X11.
There are new drivers in development, which will be open source and an open source mesa for that too but again, priority here is wayland. To be fair, I don’t know whether it will work at all for X11.
To investigate any further you could look here for instructions on how the debian image is built:
Debian image build instructions
and here for the new open source drivers and mesa:
Imgaination open source PowerVR links