This is similar to my mother's Norwegian apple cake, and I was able to convert it to gluten free simply by substituting a gluten free flour mix, 1 tsp. of xanthan gum, and an extra egg. I also cut the butter in half and added 1/2 cup milk. Delicious, fast and a wonderful way to use up extra apples!
Is 2 tablespoons of grated lemon peel correct? You can't get that from 1/2 a lemon. 2 teaspoons perhaps?