As the name implies, "gaining weight" means increasing the proportion of adipose tissue in the body. Weight gain includes not only the increase of adipose tissue, but also the increase of muscle tissue.
Human body weight comes from bone, muscle, fat, viscera and water, and only fat and muscle can increase reasonably. The significance of healthy weight gain should focus on muscles! When fat increases beyond a certain proportion, it will have a series of adverse effects on the body, and the most direct relationship is cardiovascular disease.
There are three main factors in how to gain weight healthily: reasonable diet, scientific training and regular life! However, these three sentences are really easier said than done. Many people really can't do it.