I had that same thought recently, realizing that the DPC was, in fact, a very early GPU. But Joe DeCuir created a graphic co-processor (nicknamed COPR) for the Atari 800 circa 1978 that was GPU-like. Check that out. That may be the first, at least in the video game arena.
If you can track down Joe Decuir (currently teaching at a university in the Seattle (or Portland?) area, he is usually happy to explain the old game hardware even down to the integrated circuit level.