Gaining

If you want to gain just increase your food intake.
Try to eat bigger portions at each meal.
Forcing the process can cause issues like heartburn and the like.
There is no science or magic food,pill to gain weight.
You body chemistry is unique to you and what works for some may not work for you.
I will say if you like milkshakes or ice cream have some before going to bed so you are sleeping and not putting out any energy while the calories are adding up.
Its a commitment and it takes effort to gain so thinking it will be easy is not the way it is.
Keep increasing the amount of food you eat on a daily basis and start snacking more in between meals helps too.
There will be some here that say for X amount of calories you eat Y number of pounds you will gain,this is not how it works. People that say that are not gainers and have no idea what they are talking about
By eating more your body's metabolism will slow down eventually and you will notice your weight going up but it may take a few months for this to happen so don't get discouraged just keep it up because it takes time .

Increase the size of your meals, little by little. Before long you're appitite will adapt to the larger portion size.

Add snacks throughout the day. If you want to be a bit reckless, have something sugary whenever you feel thirsty or sluggish (still make sure you get enough water of course). Before long it'll become instinctual.

Make sure to have something high in calories before bed as it'll be more likely to be converted into fat due to you're motabolism going into rest mode while you sleep.

Binge on the foods you enjoy. Sweet and salty work well, enjoy crisps then icecream after meals for example.

You don't have to consume huge ammounts of calories all at once to gain weight, just make changes to your habits like these and you will steadily gain. As long as you're exceeding the recommended daily calorie intake consistently weight gain will occur.