You need to have a high protein and carb diet. This is what worked best for me and helped me gain 25 lbs in one off season back when I played. I would eat a lot of PBJ sandwiches/carbs like potatoes and rice and a high diet of chicken/red meat like steak/ and beef/turkey. Eat 1-1.5 grams of protein per pound of body weight. When I wanted to gain weight. Went from 150-175. What I did was eat around 165 grams of protein to start and would increase the amount as I would gain weight. Drink a gallon of water a day. This helps your muscles stay full.
Get on a workout program that has you lift properly. Heavy weight, the right exercises and the amount of reps necessary to get the most out of your workouts. If you would like to get my workout plan that I just developed for weight gain, click below ⬇️ https://www.firstdowntraining.com/product-page/28-day-weight-gain-workout-plan