For a very basic weightgainer paired with a solid workout routine and diet, I would recommend either just using a simple Whey Protein Concentrate, or using our weightgainer mix called Myo Milk Formula which is a combination of wheys, casein, carbs, and healthy fats which will help you build mass:
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Also, you might consider looking into using Creatine as well. I always recommend consulting a nutritionist or physician in advance to ensure that any weight gain is done progressively and as healthy as possible