This already exists to some degree, the Raspberry Pi 5 has 2 exposed pcie lanes and people have been (kind of hackily, since some parts of the driver depend on x86) getting AMD cards to run with the pi.
I'm not an expert on drivers so I don't know entirely how close fully functioning GPU support is but in theory there's nothing stopping that from happening eventually.
As far as I'm aware specifications/implementations that have been on PCs for ages like UEFI and PCIE are not architecture specific, but it's just that a significant amount of code needs to be rewritten since it's low-level enough that the CPU architecture makes a difference.
(If someone is more knowledgeable on this, please correct me because this is all just my understanding and I could be wrong)