Skia mostly uses the CPU -- it can draw some very basic stuff on the GPU, but text and curves are a CPU fallback. Quartz 2D is full CPU. cairo never got an acceptable GPU path. Direct2D is the tessellate-to-triangle approach. If you name a random vector graphics library, chances are 99% of the time it will be using the CPU.
Skia has code paths for everything: CPU path drawing, CPU tessellation followed by GPU rasterization with special paths for convex vs. concave paths, NV_path_rendering, Spinel/Skia Compute... It's actually hard to figure out what it's doing because it depends so much on the particular configuration.
