@charlonis Only way for you, or anyone, to gain weight is through caloric surplus. Consume more calories than you burn. Try tracking them if you want to be sure, but generally speaking if you buy mass gainer powder and take a scoop daily with your normal diet you'll start gaining weight, though, those can get expensive after a while. If you don't want mass gainer powder, than try to find cheap, healthy, calorie dense foods, just search for some online and you'll find options.

@charlonis calories, no, unless you make it into a shake by adding stuff like bananas, peanut butter, yogurt, ice cream, etc. However, still optimal for muscle building and maintenance, whether you're gaining mass or not.