Even while watching youtube videos or just in desktop. I've noticed this since Radeon Crimson/ Windows 10 got installed.
It's really annoying me, since Crimson I've been fighting against high temps in my gpu. When turning manual control on fans turn on when i set a very high % (ie 80%). I've tried doing a clean install of a newer version of Crimson but nothing.
My system works fine, my CPU temp is low enough, I keep my case clean and a very good airflow, actually i never had any issue related to temps.
77°C is perfectly fine on a GPU. You can use MSI Afterburner to create a custom fan speed curve if you want it lower.