This is a game engine bottleneck, not a CPU or GPU bottleneck.
Doom 2016 is probably the worst title you could have chosen to check your GPU usage.
The Doom 2016 game engine does not allow you to generate more than 200 FPS so part of the GPU (52%) generated 200 FPS and the rest of the computing power (48%) is waiting for further commands.
I had 6700K and 1080Ti which is almost the same as you and I will tell you that you will have problems in games where 8 threads is not enough.
Mainly titles from Ubisoft like the division 2, assassin's creed origins/odysey etc. and some other games.
Overclock CPU and try to transfer the load of rendering to the GPU by increasing the level of graphic detail in games.
If you are open to extreme solutions and you are not afraid to damage the motherboard and expose yourself to potentially unproven software then you can use the modified BIOS and unlock the possibility of installing 8700K on an unsupported platform.