In a large bowl, beat the sugar, pumpkin, oil, egg whites, egg and vanilla until well blended.
In a large bowl, combine the brown rice flour, potato starch, tapioca flour, baking powder, baking soda, xanthan gum, salt and pie spice; reserve 1/4 cup for topping. Gradually beat remaining mixture into pumpkin mixture until blended. Spread batter into a 9-in. springform pan coated with cooking spray.
For topping, in a small bowl, combine the pecans, brown sugar, cinnamon and reserved flour mixture. Cut in butter until mixture resembles coarse crumbs; sprinkle over batter.
Bake at 350° for 40-45 minutes or until a toothpick inserted in the center comes out clean. Cool on a wire rack for 20 minutes. Remove sides of pan. Cool completely. Sprinkle with confectioners' sugar if desired.
Read all ingredient labels for possible gluten content prior to use. Ingredient formulas can change, and production facilities vary among brands. If you're concerned that your brand may contain gluten, contact the company.