PrPro uses the GPU as a separate item. It's not just an addendum to the CPU. So certain things are sent to the CPU, certain things to GPU if available. If you're doing color work, Warp, several things like that, the GPU will be used a lot more than it is for basic pixel decoding.
But even that varies depending on which CPU you have. It used to be a lot simpler to figure out what was CPU and what was GPU work. But with the way some CPUs have different encoding capabilities for say H.264 than others, it's almost bizarre the number of options for how it's processed now.