You're running a 1080 with a 9400f which isn't exactly a powerhouse CPU. You're CPU bottle-necked, especially if you run at lower resolutions. So just about all games will likely make your little CPU work much harder. Even on 1440p my 8 thread 3770K will run in the 90% cpu utilization range or higher on a number of games.
An easy way to tell if there's a problem with your CPU is:
- See if it's making ratings compared to other entries on CPU benchmark sites.
- Or see if it's throttling (Getting hot and slowing down) The later would indicate a bad heat sink mating.
As to your GTA V performance, it still could be a driver issue or in game setting. Games are optimized in drivers on a game by game basis. There are generic settings of course, but for specific popular games, hand optimizations are packed into the drivers for that particular game by NVIDIA/AMD. However you might have a specific in game setting which causes excess CPU usage.
BTW: How are you determining GPU usage? Video card driver overlay? GPU-z? or windows task manager?