1. Weight gain is limited by genes but still possible if your parents or siblings or not slim like you.
2. There are many causes for not gaining weight apart from genetic predisposition
3. First and foremost is to de-stress
yourself, have a good sleep --> increases the hormone level which build up the body
4. Good nutrition with high protein diet - eggs, meat, fruits, fishes sometimes even supplements of aminoacids/creatinine
5. Muscles wont build unless there is a need. So going to gym regularly will increase muscle mass and body energy turn over. It also de-stresses your body and soul.
6. Foods with high glycemic index
like watermelon, icecreams, corn flakes increases your appetite by indirectly lowering your blood glucose
through sudden insulin
7. More important is : sudden weight gain say more 10-15 kgs within a year is not good for your health, as your bones and other supporting structures remodel slowly which might take 6-8 months to accomodate your recent weight gain. so its better you give a break for 3 months and continue the cycle.