Actively_Funny is right.

And my short answer would just be:
WHOLE FOODS!!! And Animal Proteins (mostly fish and chicken).

and LIFT BIG TO GET BIG.

you won't find too much help here as everything is geared towards weight loss.

Mate you should eat more and more foods.This food mostly contained on protein and carbs.Eggs,meat,milk,banana etc are good for gaining weight.You should also do exercise for gaining weight.

Without high protein diet, muscle growth, muscle maintenance and workout recovery will just not be happening as effectively as it should. It also keeps your hunger in control. So, you should include animal and dairy products in your diet, within reason.

Foods for gaining weight
 

Hello, there are so many foods for weight gain.
Eat whole grains, such as oats and barley,
Nut butter like almond, peanut and cashew which have a calcium & proteins.
Choose lactose-free milk or soy-based equivalents
Fish is a very nice source of protein and essential nutrients, like we get B vitamins and zinc from fish.