If they're both at 100% use then it sounds like they're well matched for the game. The gpu doesn't really take the load off the cpu, they process different things. The cpu handles the physics such as character movements and objects etc, the gpu handles the 'painting' of the screen. Things like textures, shading and so on.
Have you checked that the cpu is reaching full speed and not stuck at 800mhz or anything? What temps is the cpu reaching and what are your fps in game?