In gaming, bottlenecks are not simple. GPU and CPU load are independent of each other, largely.
If a game can only run at 40fps on your CPU, but 60fps on your GPU at low settings, 40fps at medium settings and 20fps at high settings, then:
-At low settngs, you'd get 40fps, because the CPU is "bottlenecking".
-At medium settings, both CPU and GPU are capable of the exact same framerate so there's no bottleneck
-At high settings, the GPU is the bottleneck and you'd only get 20fps
What complicates things is that the CPU and GPU loads change from game to game and from scene to scene. Some games need more CPU power for the same framerates, while others are very light on the CPU. Also, increasing or decreasing graphical settings can put more or less work on the GPU, while there are generally no settings that can affect how loaded the CPU is. If you're getting slow framerates and your GPU is the weak link you can simply drop graphics, but if your CPU is the weak link, there's nothing you can do about it but replace the CPU. For this reason I tend to recommend people spend proportionally more on CPUs.