For weight gain it is very necessary to eat small and frequent healthy meals along with slow exercise.
Start regular yoga
/ pranayam every day undersome someone's supervision. Eat 2-3 servings of fruits everyday. Avoid all junk food items. Include high fiber, high protein like egg whites, whole pulses, sprouts, Nuts in your everyday diet.
You can also get in tough with a Dietitian to get a proper diet chart for weight gain.