Banana with milk is constipating me, what should I eat to gain the weight?

Calories. To gain weight simply eat more calories than you burn off. Try to maintain a balance of proteins, fats, carbohydrates and fiber to meet your nutritional needs. Increase water and fiber if constipatopn occurs.
Foods to help with. Healthy weight gain: almonds, cashews, pecans, sunflower seeds, flax seeds, walnuts, hummus, avocadoes, granola, trail mix, dried fruits, extra virgin olive oil, canola oil, protein shakes, protein bars, lean meats, poultry, fish, smoothies (with added silken tofu or powdered milk), greek yogurt (full –fat), pasta/ bagels ; use of juice rather than soda. Some people add whey proteins.