I don't know why you are too concerned about this number unless you're looking into CPU bottlenecking. CS:GO is really not a GPU intensive game nowadays, and your GTX 1080 barely has to lift a finger to run this at max settings. That's a good thing. Titles like PUBG are more demanding, which is why you're seeing higher usage % numbers.
Think of it as having a lot of headroom for games in the future. Sure right now your GPU is only being "30% utilized" on games like CS:GO, but all the high end titles that come out in the next couple years will be able to fully utilize your GPU and you have a lot more room to do so.
If you're concerned that you're not getting value out of your GPU because it's not being fully utilized with the games you play, well then you're kind of right. You don't need a GTX 1080 to play CS:GO at over 200 fps. But you made a great investment for games you'll play in the future.