To make this Spanish Coffee Cake gluten-free, substitute the all-purpose flour with a 1:1 gluten-free flour blend that is suitable for baking. Ensure the baking powder and baking soda used are certified gluten-free. The rest of the ingredients, including the sugars, oil, egg, and walnuts, are naturally gluten-free, but it is recommended to verify all product labels to avoid cross-contamination. Baking time remains the same.