Step 1

Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch pan; lightly flour with gluten-free flour.

Step 2

Whisk sorghum flour, almond flour, rice flour, tapioca flour, xanthan gum, cinnamon, baking soda, baking powder, and salt together in a bowl.

Step 3

Whisk sugar, vegetable oil, and eggs together in a separate bowl; stir into flour mixture until incorporated and batter is thoroughly combined. Fold in apples and walnuts. Pour and press batter into the prepared pan.

Step 4

Bake in the preheated oven until a knife inserted into the middle of the cake comes out clean, about 1 hour.