Pretty much all of the rendering pipeline runs on the GPU, in compute shaders. In most 2D renderers, the CPU is doing a lot of the work (tesselating vector shapes into triangle meshes, computing bounding boxes, etc). In piet-gpu, the CPU just encodes a high-level, declarative description of the scene (semantically similar to SVG) in an efficient binary format, and the GPU does the rest.

Regarding hardware support, it needs a GPU capable of compute shaders (which rules out a bunch of the older or very low end mobile chips). There is a hardware abstraction layer which currently supports Vulkan, Direct3D12, and Metal, but I believe D3D11 could be added, as well as other lower level interfaces (I took a look at deko3d and was tempted to use that as a way to port to Nintendo Switch).