Your GPU mainly gets used whenever shaders come into play.
A lot of things could still be pushed over to the GPU but that would require even more extensive changes than probably EEP did (which basically just shuffled stuff around a bit). Currently however the CPU prepares a lot more than necessary. The Viewer naturally does a lot of rendering on the CPU because A: some of the Viewer code parts are quite old and B: some information needs to be prepared to be used by the GPU.
GPU usage highly depends on a few aspects such as used rendering features and your resolution. I suppose i'll have to repeat myself here:
