i gained weight in the form of lean muscle mass by having 5-6 meals a day with the focus of each meal being protein, essential fats, and some carbs. i worked out like crazy though, pushed myself beyond what i could lift and had people spot me for most of my sets.
whey protein isolate is apparently the easiest protein to digest, especially when mixed with water. generally i had 1-2x shakes per day. i also took creatine.