01 - Preheat oven to 350°F. Grease a 9x5-inch loaf pan and line with parchment paper.
02 - In a medium bowl, whisk together gluten-free flour, cinnamon, baking powder, baking soda, and salt until well blended.
03 - In a large bowl, beat oil and brown sugar until combined. Add eggs one at a time, mixing well after each. Stir in applesauce and vanilla extract.
04 - Gradually add dry ingredients to wet mixture, mixing until just combined. Do not overmix.
05 - In a small bowl, combine ⅓ cup brown sugar and 1 tablespoon cinnamon.
06 - Pour half the batter into the prepared pan. Sprinkle with half the cinnamon mixture. Add remaining batter, then top with remaining cinnamon mixture. Use a knife to gently swirl through batter for marbled effect.
07 - Sprinkle chopped nuts over the top if using.
08 - Bake for 40 to 50 minutes, until a toothpick inserted in the center comes out clean.
09 - Cool in pan for 10 minutes, then transfer to a wire rack to cool completely. Dust with powdered sugar before slicing if desired.