Hello, for weight gain you need to consume more calories then you use. Thus try to eat 4 - 5 meals so that it's easier for you to eat more. Consume more of protein rich foods like dals, sprouts, milk & milk products ie cheese, paneer, milk, lassi this will make your meal calorie dense. Also try to include fruits and vegetables.consume one banana daily. If you are non vegetarian then include one egg daily and also include fish, chicken thrice a week.
Inbetween meals you can snack on dried fruits too
For personal and well planned diet chart massage in personal.