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.
The COPR may have been incorporated into the ANTIC chip, but I don’t remember the 800 hardware to that extent. I recall that it had a rudimentary micro-coded instruction set arranged as a display list that could blit sprite data with simple, real time decision making.