I go to equip, I have a few gems but it doesn't give me the option to select one.
You have to match the color of the gems of course. But you have to go to avatar in the Home Screen then select stock->tank->avatar-> then you will find your gem slots to input your gems in the appropriate color, then after you have selected->equip, you !must! verify you will discard your old gem If there is already a gem in place. Hope this helps.