If you are one of those people who just can't gain weight, try a weight gainer to add extra calories to your daily diet. Calories are key to gaining weight.
Other things to consider in a gainer are carbs and fats. You need carbs to help protein absorption, and you know you need the protein. Fats are definitely your friends in a gainer (though not saturated fat), as they seriously up the calorie content. A general rule of thumb is twice as much protein as fat, and twice as many carbs as protein.
For any bodybuilder, protein is the key to a good diet. For a hard gainer, the best way to get that protein is in the form of a gainer.