Mass Gainer contain loads of carbohydrates. You can't build muscles on carbs. Muscles are made of protein and fat. Whey protein is a better choice for you. Only soy protein would make your hormone goes haywire. Whey protein is perfectly safe for long term consumption.

Also you need to understand that total calories dictate that how much weight a person gains or loses; macronutrient ratio dictate what a person gains or loses.

In you case to loss weight you need to reduce you calories while you wanna loss fat & build muscle, your protein ratio must higher than carb.

My suggestion to you is go hi protien, hi fat and restrict carbs diet,
For Protien Supplement i suggest the Gaspari Myofusion.