optimum weight gain depends upon proper digestion and absorption.  if it is on right track, diet change will be enough for weight gain.  I can suggest dehaposhan yamakam 5 ml twice after food with cuminseed water

Your body requires a cleansing therapy initially.. For that you need to drink 12 glasses of water and keep a healthy diet plan.. You need to add veggies and fruits in abundance in first week. Drink 2 glasses of water in empty stomach.. Wash your face thoroughly and then steam your face daily one time These steps will help in clearing acne and promote proper digestion.  Once your digestion improved you can try milk shake recipes . Each stage requires ayurvedic medicines too.. It can be said after proper history taking.. You need to check thyroid profie too