Do you have vsync enabled? If you do then your gpu will only use what it needs to handle your monitors refresh rate. Your refresh rate depends on your monitor 60/75/100/120/144/165hz and your gpu will match this with vsync on 60/75/100/120/144/165fps. The lower your refresh rate the lower your gpu usage will be when vsync is enabled. What monitor do you have?
Check what individual cores are boosting to on the CPU. The 3600 should have at least one core boosting to approaching 4.2 with the rest not too far behind.
Have you got any kind of eco mode / power saving enabled anywhere ?