Gaining

It is not really that easy, but a big mix of all kinds of foods will help and overeating of course, stretching the stomach to hold more, more calories to digest and fatten you up.
I have made great experiences with taking a small glass of olive oli every 8 h or so, not too much, around 25 ml to 50 ml, because the body cannot digest that much fat at once, but the little portions again and again has fattened me up.
I take the olive oil in the mouth really quick and then quickly flush it down with orange juice or something, so that the tongue cannot taste the pure fat, which normally is not pleasant.
You can also take cream, that is not pure fat, but tastes better. But there might be some health implications about the bad kind of animal fats that olive oil does not have.

Buy carbo gain, drink 1l of heavy cream and add carbo gain or ensure plus, do this as frequently as possible (I've been doing it 3x per week for the last month and have gained 35lbs)

I agree with others above, but I'll add that for me ice cream was a major contributor to my gain. High in calories, easy to eat and delicious.

Definitely this. My belly is currently the result of ice cream, ice cream and more ice cream.