Problem with FPS?

Defalt

Hello. 
So yesterday I've bought a new Predator PC.

 

Acer Predator G-137:

GPU: nVidia GeForce GTX 1060 6GB Version 382.05

CPU: Intel(R) Core(TM) i5-7400 CPU @ 3.00GHz

8gb

 

For some reason when I play GTA V or Watch Dogs 2 Doesn't matter if I play on Lowest or Highest settings, the FPS are on 45 fps if lowest settings and 20-30 fps if on highest settings. As far as I know this PC should be handling those games at high settings no problem. And for e.g., GTA V when I set it on highest settings it's only taking up 3.2gb of the GPU out of 6gb I don't know if that is a normal thing but is there anyway to imrpoove those FPS when playing on high settings?