The GPU is not simply another assistant to the CPU for basic playback in Premiere. It is 'reserved' for things on the GPU Acclerated list of effects, such things as color work and resizing like Warp. So unless there are color changes or resizing, the GPU may not be used much.
And some H.264 and RAW formats do get some GPU tasks assigned also, but I'm not the one to help with that.
So ... it might be that with different settings you would see more GPU use for playback, but ... it might not. I can't say more as you didn't say what formats/codecs are in use, nor what effects.