What is your current weight?
If u r tall and thin, it is much decided by your genetic constitution.
For weight gain you can increase your calories intake.
Take some good fats-omega-3
in dry fruits ,walnut,flax seeds.
Carbohydrates should also be increased but take whole grains like whole wheat preparations.
Take meal 4-5 times instead of 3 times.
ayur.-aswagandharishta 2 tsf twice after meals.
Wish u good health