Real-time shading/rendering is essentially a massively parallel operation -- the exact type of problems that GPUs (and supercomputers in general) are good for.
In fact, many of the top supercomputers today use GPUs.
In fact, many of the top supercomputers today use GPUs.
Also, I would assume the reason behind supercomputers not being good for gaming is that they are parallelized in ways other than the chips on the GPU (entire machines are networked together via various interfaces). The software behind distributing the processing between several machines, or whatever aspect makes it super, is probably what limits the ability of supercomputers to run video games :0