The PGC was perhaps the first GPU for PC systems. It had its own 8086 processor on board, which would accept graphics commands through a ring buffer shared with the host system, and the 8086 would translate those into frame buffer operations it needed to perform. A few years back some clever git found an exploit to allow arbitrary code to be run on the PGC's 8086.
The Tandy 2000's color graphics was not spectacularly great by any means, nowhere near comparable to high-end solutions like the PGC. It was a 96 KiB framebuffer divided into 3 bitplanes, only one of which was mapped into system memory at any one time. You changed which plane was accessible by poking one of the video registers. There were also some palette registers that let you change which 8 (out of the standard IRGB 16-color set) were visible on the screen at once. Since drawing a full-color display meant 3 plane selects followed by 3 writes to video RAM, it was slow to produce such a display and there were "temporal color fringe" artifacts as the writes to different planes were momentarily visible. But it was still great for CAD and DTP applications, especially for the money, and good enough that Microsoft used it as a development platform to build a color GUI for Windows, while Macintosh was still committed to producing the best looking, black and white display they could for the money.