Can I take this multivitamin, mineral and amino-acid supplement powder to help with weight gain?

It's a fair question, because people often assume any nutrition powder will automatically add weight. This one is really a blend of vitamins, minerals and amino acids meant to fill nutritional gaps, not a dedicated high-calorie weight-gain formula, so on its own it's unlikely to move the scale much. Actual weight gain comes down to eating more calories than you burn, ideally through a balanced diet, with a supplement like this supporting that effort rather than replacing it. A doctor or nutritionist can help you build that into a proper plan.