Welcome to HCM, To gain weight quickly eat late at night just before bed. Eat larger portions of carbohydrate and fat. The two together at the same meal will promote fat gain. Nuts + bread; margarine + potato, Full fat milk plus fruit smoothies with peanut butter and chocolate added in.. Eat large meals, larger as the day progresses. Eat healthfully, but do everything just the opposite of someone trying to lose weight. Snack on trail mix or things high in fat/calories like cheese/crackers, nuts and dates, these things. Putting on muscle increases weight. You are currently eating carbohydrates to gain weight. A carb = 4 calories per gram as does protein Fat weighs in at a whopping 9 calories per gram. Use extra virgin olive oil to dip bread in (healthy fat +fiber and carbohydrate from bread), so use healthy fats when you increase your fat intake. I hope this works for you. Kindest regards, Kathryn Shattler, BSc, MS,RDN