It will be years away at the least. Do you have any idea how much heat the current batch of high end GPU puts out? I don't want to end up looking like Sandor Clegane (the Hound).
Maybe if they do liquid cooling, but that ain't Apple's style. Too bulky.
They take a lot of power too and one GPU isn't enough these super gaming rigs have multiple GPUs with multiple cards. Personally I'm not into any of that today.