Suggest Ways To Gain Muscle Mass

Hi doc. I'm 20 year old, 5 foot 11, weight 12 stone, non smoker, drink once every 2 to 4 months. Mother and father both regular built but uncles on both sides have a big gut which I have but that is not my concern, im struggling to gain muscle mass everywhere else. I've been eating all healthy, porridge, fruit, veg and meats. I also consume 3 protien shakes a day. I play Gaelic football 3 times a week and go gym every other day. I've been doing this for a month and a bit with no results.. What is it I'm doing wrong?