your gpu (usage) should be at 100%. it's designed to handle only the 3d environment you're currently working with (the game) if its going full tilt, that means it's doing it's job and isn't being held back by other components in your system or other software related setbacks. the cpu handles not just the game, but EVERYTHING your pc is doing at all times. driver software for kbm, audio suites, bluetooth stuff, the OS itself, web browser, discord, etc. if cpu goes to 100 you have a problem.
If your gpu usage falls below say 80-90% then something is holding it back from doing it's best. maybe the game isn't demanding enough and you need to turn up the graphics a little, maybe your cpu is struggling to send instructions to it quick enough so you need to kill a few background things or even just set view distance, effects quality, or enemy/entity count (in any game with these options) to a lower setting since those mainly tax the cpu.