Gaining

Go a long time without eating, then binge, your body will store more calories as fat in case it starts to "starve" again lol. Also, eat a lot of calories right before you sleep. There are a lot of good advice posts here as well.

Vitamin B1 tablets (1 a day, at least 50mg) for increased appetite and better use of the food you eat.

Chocolate is as calorie-dense as anything you can eat, but you can make do-it-yourself weight gain shakes with anything from melted icecream to unsalted peanut butter to instant cake mix.

Have big, late meals so you store the weight while you're sleeping instead of burning it off.

And enjoy yourself. Don't let eating become a chore.