P.S I am using latest drivers.
Windows 8.1 64bit
Low GPU usage usually indicates that GPU performance is being held back by either the game engine, API or a system limitation such as a CPU bottleneck. You can sometimes overcome these limitations by either using a high resolution, or higher image quality settings which will see GPU usage increase. Or you would need to upgrade your CPU or increase the speed of your system memory, which may provide more performance and increase GPU usage.
CS:GO is a very CPU heavy game as the CPU is performing many calculations while playing online with other players.