Preheat oven to 350°F (175°C).
In a large bowl, whisk together gluten-free flour, xanthan gum (if using), salt, and sugar.
In a separate small bowl, dissolve yeast in warm milk. Let stand for 5 minutes until foamy.
Add yeast mixture, olive oil, apple cider vinegar, and egg whites to the dry ingredients.
Mix on medium speed until well combined, about 3-4 minutes.
Pour batter into a greased loaf pan.
Let rise in a warm place for 30-45 minutes, or until doubled in size.
Bake for 45-50 minutes, or until golden brown.
Let cool in pan for 10 minutes before transferring to a wire rack to cool completely.
