CPU, Most if not all renderers use the CPU. However games, on screen editing use the GPU, this can be seen the specs when OpenGL, DirectX, etc. are mentioned. There has been break throughs in using the GPU to handle some of the rendering process in a few renderers, but its still new technology, and from where I am sitting, I personally believe it will be awhile before Bryce starts using the GPU for rendering.
If you change the view mode from wire frame to one other other modes without rendering, you can get a glimpse of the GPU at work in Bryce, but as far as I am concerned the view is quite crude compared to a render.
But as I say the technology is available, its just the programers need to take advantage of it, assuming, there is little to no cost for using said technology.