Hi, when you want to gain what you need to gain is muscle. Focus on having good quality protein food sources like milk, curd, pulses, sprouts, egg, chicken, fish etc. Try to eat small meals every 2 hours. Make your meals more energy dense by adding ghee to roti or rice. Fruit milkshake as snack or a handful of mixed nuts. Drink atleast 8 to 10 glasses of water daily. Include some strength training in your exercise routine regularly.