My 3400g gpu this oscillates 100% with no open game.
I bought it 3 weeks ago and a week ago I started to be missing when I bought a new 165hz monitor(and everything ok until here) and updated windows. I also did a dust cleanup.
The performance of the games decreased enormously from one day to the next
battlefild 5 step from 65 fps to 29, minecraft step from 300 fps to 60fps
Temperature 25 C° to 45C°
This tech site gives some good tips on how to find the cause of high GPU usage during idle: https://www.faceofit.com/fix-gpu-usage-100/
As mentioned by the linked article use Process Explorer to find out which program is using the most GPU usage.