So, the thing is the following, from what I gathered so far: Your GPU is fine, as it is running on 20 % load. Seeing as the GPU can easily handle 200+ FPS I am not surprised.
I would recommend you to test your CPU/GPU on two different kinds of games:
Games are not the same. You have CPU heavy games, as for example Civilization 6, etc., and you have GPU heavy games, like the Witcher 3 and stuff.
If you really want to find out, if your CPU is indeed bottlenecking your performance, check two different kinds of games, as mentioned above, and compare the results.
You should get a much higher performance on Witcher 3, for example, than in Civ 6, if your CPU indeed is bottlenecking.