[1] Maybe I've just been blindly ignorant for 30 years, but as far as I could tell, 'GPU' seemed to emerge as a more Huffman-efficient encoding for the same thing we were calling a 'video card'
In the context of the discussion, the point is that you don’t need high-powered graphics hardware to achieve a fast GUI for most types of applications that WPF would be used for. WPF being slow was due to architectural or implementation choices.
GPU-accelerated GUI usually refers to using the texture mapping capabilities of a 3D accelerator for "2D" GUI work.