Well, it depends on what you're playing.
There's a number of things going on during a computer game.
The GPU draws the things going on.
But the CPU calculates what is going on.
So what you are playing has a lot of things going on in the CPU field; Calculating . . . position, action, speed, physics, anything A.I. related etc, you might be running into CPU limitations.
Now, the things being calculated might not be in your field of vision: GPU doesn't have to draw anything: So there's no GPU problems.