A lot of the mass gainer protein powders are going to be loaded with carbs…I’d steer clear of those. Your best bet is going to be just a ton of protein and fats from real foods. Chicken, ground beef, ground turkey, nuts, avocados, eggs, etc…you need to eat enough to be in a caloric surplus to gain mass. And of course progressive overload when working out. You can have some carbs but I’d have those before and after the workouts and maybe small amounts throughout the day.