Gaining weight properly is not a matter of gorging yourself on as much food as possible, throwing aside all concern for the quality of the food you’re consuming. Instead, aiming for 1 to 2g of protein per lb of bodyweight, bump up your daily overall calories by 500 until you begin to gain weight at a clip of 1 to 2 lbs per week. The other critical factors here are frequent feedings, consuming nutrient dense, protein rich whole foods every 2-3 hrs and food choice—selecting complex carbohydrates, healthy fats, lean protein and fruits and vegetables loaded with vitamins and antioxidants as the backbone of your nutrition plan.