1. Dissolve sugar and yeast in 1 1/2 cups warm water in a small bowl; let stand 5 minutes.
2. Weigh or lightly spoon flours and cornstarch into dry measuring cups; level with a knife. Combine flours, cornstarch, xanthan gum, baking powder, and salt in a large bowl, stirring with a whisk. Add yeast mixture, oil, vinegar, and eggs; beat with a mixer at medium speed 3 minutes or until well blended.
3. Spoon batter into a 9 x 5-inch loaf pan coated with cooking spray; smooth top of dough with a spatula. Cover with plastic wrap coated with cooking spray, and let dough rise in a warm place (85°), free from drafts, 1 hour or until dough reaches top of pan.
4. Preheat oven to 375°.
5. Bake at 375° for 45 minutes or until top is golden brown and loaf sounds hollow when tapped. Cool 10 minutes in pan on a wire rack; remove from pan. Cool completely on wire rack.