A gainer is mostly maltodextrin with some whey or milk protein added: a way to drink several hundred calories when eating them is the problem. The carbohydrate is fast and the glycemic load of a full serving is high, so it suits people who genuinely struggle to gain weight and suits almost nobody else. Read the serving size on the tub — a "scoop" is often 100-150 g, which is a meal, not a supplement.