Gaining

Try the fasting thing:
One day starve, try to eat nothing and drink only water.The next day gorge all day as much as you can.
Do this 2 - 3 times a week and you will definitely gain

Keep eating and stay sedentary — it’ll happen fast enough:-)

Try to time your eating. Make sure you slam in a ton of calories early in the day. For me, if I eat a good sized breakfast, lunch is gonna be a big one. Also try getting in calories late at night. A big serving of ice cream before bed will turn into fat since you have no chance to burn it off. Then when you hit that with the big breakfast the next day, your body will have taken in loads of extra calories that it doesn't need.

3-4 weeks of heavy cream cake mix shakes every night before bed helped me push past my plateau. within a few days a began waking up absolutely ravenous, but would wait until mid-late morning before eating my first meal of the day.

constantly snacking between meals also helped. i made sure there was both salty snacks and sweet snacks in reach while working so i could mindlessly graze between meals.

and finally, i made sure i was drinking lots of soda and fruit juice. liquid calories increase your daily calorie intake by a lot.