Combine the flour, salt, baking powder, baking soda, cinnamon, and xanthan gum. Add, along with the oats, to the butter mixture. Mix just until incorporated.
Stir together the 1/3 cup flour, apples, nuts, and raisins. Fold into the mixture.
Spread into a greased 10 x 15 inch jelly roll pan.
Bake at 350° for 25 minutes.
Set the pan on a wire rack to cool before cutting.
Recipe by The Gluten-Free Homemaker at http://glutenfreehomemaker.com/apple-bars/