100% GPU usage is what you want to have, it means the GPU is giving all it has and you definitely want that.
Quick explanation; the performance of a system while gaming is the mix of CPU and GPU performance, the result is your FPS, disk access has an impact too but only for 2 or 3% . What you experience is called a GPU bottlneck.
In a game the CPU is responsible of pretty much everything that happens in a game, from the calculation of your health to the position of buildings or what the NPCs do, the GPU only draws wireframe shapes where the CPU told it to do so, throws textures on them and then applies the post-processing effects.
The bad bottleneck happens when you pair a powerful GPU with a weak CPU, if you monitor your GPU usage you will notice that it doesn’t reach 100% usage (between 98 and 100% is ok). If your GPU is below these 98%-100% it means that it’s waiting for the CPU to finish its part of the job. In other words your CPU is not calculating fast enough its part of a frame, NPCs postion, etc… and the GPU is waiting on the CPU to finish its part of the job; render the frame calculated by the CPU and send it to the output. The lower the GPU usage and the higher the CPU usage will be. This can ultimately result in a slow responsive system because the CPU is forced by the powerful GPU to keep pace with it and the CPU needs to allocate pretty much all its ressources to the game. With a poorly coded game you can end up with 100% CPU usage and your system can even hang or crash.
The good bottleneck is when a CPU is capable of keeping up with the GPU, it means it’s fast enough to calculate the position of the NPCs, where the buildings need to be drawn, and has to wait on the GPU to finish its part of the frame. The result is 100% GPU usage and your system is giving all it has, your only way to get higher FPS in that scenario is to have a more powerful GPU or overclock it.
A few years ago I tested an i7-920 (2009) with a AMD RX 580 8GB, the GPU was way to powerful for that CPU and the GPU would run at 100% for a few seconds then drop into the 30% usage because the CPU couldn’t follow the pace, Witcher 3 was just too demanding for that CPU and the GPU was sitting there waiting for the CPU to finish the calculation of the frame.
When you’re GPU bound (GPU bottlenecked) there’s nothing to do beside switching to a more powerful GPU in order to get more frames.
When you’re CPU bound (the worst scenario) the game will feel choppy when there’s a lot of things happening on screen. To solve the issue you can either switch to a more powerful CPU that can follow the GPU pace or increase the resolution. Increasing the resolution will be more demanding with the GPU, it won’t be able to generate as many frames as at a lower resolution, the CPU will be able to keep pace with the GPU and the gaming experience will be smoother.
We humans notice lags and FPS drops more than constant low FPS. If the combination of CPU and GPU gives you a constant 30 FPS the game will feel smoother than if you drop from 100 FPS to 20 FPS because the CPU can’t keep up.