I would only add a mass gainer if you cannot eat more. Getting calories in via liquid will be less filling and help with calorie requirements. However, if you do decide to go with a mass gainer, make your own. It is so much cheaper and you can customize the macros to fit your goals. Start with a carb base, oatmeal, sweet potatoes, pumpkin, etc. Add a protein source, protein powder or even Greek yogurt. Add a fat source to increase calories, coconut oil, any type of nut or nut butter. Milk, water or a milk alternative.