The gelatin glaze itself is almost flavorless aside from general sweetness, and a very faint chocolate flavor if you use white chocolate. After the excess runs off, the mirror layer is actually pretty thin, all you taste is your crumb layer (which is absolutely necessary to achieve a smooth finish).
That being said, a good looking and controlled mirror glaze is not an easy thing to achieve, and I would recommend doing test cakes before you apply it to the "hero" cake. For practice you can just buy a cake with a similar texture to what you are going to make (poundcake, angel food cake), some ready frosting for the crumb layer, and apply your mirror glaze to that.
You can also make mirror finish with pectin instead of gelatin – pectin tastes kinda fruity. But you will need a recipe that uses pectin – it's not a 1 to 1 substitution.
Vlad