To gain muscle, not fat, you need physical activity and balanced nutrition and rest (everything is important!).
Eat better, like in a sanatorium, 5 times a day.
Each meal should contain 15-20 grams of high-quality protein (after exercise, 25 grams of protein is better).
Be sure to eat vegetables (especially from the green group) and drink water (about 30 g per 1 kg of body per day).
You can add good vitamin complexes.
It depends on what exactly you want to gain kilograms. You have two choices:
-Get fat - eat more high-calorie foods with a smaller volume, for example, cakes, fried bacon, fatty meats, in general, what contains the maximum amount of fat. But It will bring you a lot of health problems and an imbalance in your figure - your belly will begin to grow (you are a man?) While everything else will be much thinner. Another great option to gain fat (and weight, respectively) is to drink sugary drinks like cola, forfeits, etc.
-To gain muscle - eat foods rich in protein - chicken breasts, eggs, cottage cheese, squid, low-fat fish and go in for sports, then your figure, although it will acquire excess fat, but in a minimal amount, and so basically it will be muscles. Do only strength training and do not increase the calorie content of the basal metabolic rate (which can be calculated using a body expert calculator) by more than 10-15%
I have almost no appetite, I melt before our eyes. And I used to gain 75kg freely, it was normal, with an increase of 175.