That's the exact definition of a CPU bottleneck.
i have the same problem with
Xeon w3680 3.33 GHz 6 core
Gigabyte gtx 670 windforce 3x OC 2GB
10 GB ram 1336 Mhz
625 PSU 80 plus bronze
and the cpu in like 30-60% usage and gpu is on high 80 gpu usage with 100 fps and in low is 50% with 100 fps too so is that a bootleneck?
but it's not at all games so games like gta v the gpus usage 95% with +80 fps and cpu stays at 50 % usage
but in online mode gpu usage is 60% and cpu 30% ??