Gaining

White bread and butter. Very economical and fattening. Diligently eating 10 slices per day.Pancakes every morning.Lots of syrup and butter.Make them yourself. Shop for cookies and honeybuns or snack cakes at Dollar Tree Family Dollar and Dollar General. Also there are good deals at Walmart.Eat in the morning until you're stuffed to stretch your capacity.Get full right before bed and sleep and gain.
Also get the largest size coffee at convenience stores and mix it with tons of straight cream and flavored creamer .Almost half of the 24 ounce cup should be fattening cream and creamer.That can get you 700 calories if you mix it right.
Hope that helps.

Drink heavy cream a couple of times a day or eat a bowl of ice cream before bed.

Worked really good for me.

Let us know if it helps any.
Ice cream puts weight on me easily so it may do that on you YMMV of course.
I'm not an active gainer anymore so I don't eat ice cream much these days.