Mass gainer shakes probably work, but they are stupidly expensive for what they are (just some extra carbs). Add some more rice/pasta/potatoes to your meals and you will get the same effect. Weight loss is simple; Eat less. For weight gain the exact opposite counts; Eat more! You don't have to go overboard and a "healthy" rate of weight gain would be about 2-3 lbs a month. Do that for six months and you will be considerably heavier.