Gaining

Key is shopping when you're HUNGRY. You'll buy more than you would otherwise.

Drakes products. Yodels. Ring Dings. Funny Bones
They're So delicious and yet to me at least they seem light enough that I could eat an entire box (1800+ calories ). Without feeling painfully Stuffed. Since I am not a gainer I will never do it. But I am sure I could. Drakes products are also so good that You will find yourself wanting "Just one more " over and over.
Entenmans is also a great product line. Danish. Cheese Buns. Sheet Cakes. All yummy and Fattening!
And for a quick calorie blast on the go. Most convenience stores have iced honeybuns ranging from 500-760 calories Each.
And for ultra cheap gaining. Plain white bread with plenty of butter or margarine on both sides. Not the tastiest most exciting food - but it doesn't taste bad either.

Fruit juices *nice and calorie dense while still containing nutrients*

And don't forget about veggies. They're still very important even if you're gaining.

Don't make junk food your go-to. That's how you get the fast food brain rot (FFBR). Your brain needs nutrition to function, and when you don't have it, it gets dumb. Like ... really dumb.

To say nothing of the rest of your body shutting down and crumbling due to malnutrition.

Here's an article about healthy gaining:
healthline.com/nutrition/18-foods-to-gain-weight