It would be cool to see some raspi hats you could plug a GPU into, though unsure of how practical or feasible that would be. Todays graphics cards are tomorrows e-waste, perhaps they could get a second life beefing up a diy raspi project like this
anyhow the GPU would sit on a small PCB that would connect to an even smaller PCB in the PCIe slot on the motherboard, via USB3 cable. My point here is merely that whatever PCIe is, it can be transported to a GPU to do work via USB3 cables.