I bake exclusively gluten free, and you cannot tell my baked goods from the gluten ones. I don't use commercial mixes as they don't produce the taste I want. But, suggestions: If your mix has xantham gum already in it, don't add any more. Even a little bit too much or too little can ruin your cake. GF cake mixes turn out better than general purpose flour mixes. Try lowering your oven temperature by 25 degrees and bake a little longer. That usually stops the shrinking.