I am of the belief God has given us things of the earth to help in losing weight naturally, and gaining it in the same manner. So I would suggest things like:
Steaks. They contain a ton of protein and iron. But try to get the ones with marbled meat patterns because that has more fat calories.
There's Natural Organic Peanut Butter. Again, packed with protein and fats, making it a great choice for people trying to gain weight in a healthy way.
One way too is to change your milk intake from non-fat, or 1%, or even 2% and go whole milk. Even adding it to recipes etc. Its higher in Vitamin D and A too. It is only about 60 Calories more than the other milk types, but that adds up fast per glass size.
Here's a great thing, we all have heard an apple a day keeps the doctor away, but tropical fruit will blow you up like a Macy's day Parade balloon! Smile. No, but seriously it is awesome tasting and adds weight fast. Things like: Mango, Papaya, even Bananas, and pineapple are amazing choices which include natural sugars and give lots of energy.
And don't eat junk food fats but eat good fats, like Avocados, and eat natural granolas too...You got this! Just get to eating and watching a lot of TV. NO! That part I am not suggesting....Smile. The point is stay healthy along the way.