You gain mass by eating protein AND exercising.See an advosir at a local fitness club. That is what they do. They can help you out.
It depends on your calorie intake and your activity level. Your body type and genetics have a lot to do with how you process the food you eat. You need to talk to a trainer that is also a dietitian/nutritionist.
You need to maintain a high protein diet and excersize especially weight lifting to put on muscle.