Hi

 Gainers helps you to increase your weight as well as your muscle mass. It has both carbs as well as protein in it (depends on the scoop size as different brands have different scoop sizes). Any good gainer is a blend of six different types of proteins,complex carbs and digestive enzymes which helps in muscle recovery and helps in gaining weight. Gainers are generally consumed in bulking phase.

But if you want to build only muscle mass without gaining weight then choose whey isolate post workout. Isolate is a pure form of protein, it will help you in building muscles. It does not contain any carbohydrate and fat. But diet is equally important in this. Do consume lean proteins in the form of chicken,eggwhites and fish along with complex carbohydrates and good fats like oats,quinoa, flaxseed etc.