dude the gaining and losing of weight is extremely simple. Your body burns a certain amount of calories a day. A number you've probably heard before is 2,000. Everyone burns a different amount of calories due to many different factors for example size of the person and metabolism. So lets say your body burns 2,000 calories a day and one day you eat 1,800. You will lose weight that day because you consumed less calories than you burned. If your body burns 2,000 calories in a day and you eat 2,200 then you will gain weight. You must have been eating less than you burn. If you want to gain weight then eat more than you burn, that simple.