Will Gaining Weight Fill The Sunken Cheeks With Muscle?

No,if sunken cheeks are hollows below the eyes then the skin has lost its elasticity,it can be improved with facial exercises to increase muscle mass or implants or fat injections to fill the area . If the cheeks are hollow around the mouth giving a gaunt look then a little bit of weight gain may improve the look.  Muscles do not become larger by increasing size of fat cells [gaining weight] only exercise can do this ,with limited results.