In a small bowl, combine the warm water, sugar, and active dry yeast. Stir gently and let it sit for 5 to 10 minutes until foamy.
In a large mixing bowl, whisk together the gluten free all-purpose flour, xanthan gum (if using), and salt.
In a separate bowl, beat the eggs, then add the melted butter and apple cider vinegar. Mix well.
Pour the yeast mixture and the egg mixture into the dry ingredients. Using a stand mixer fitted with a paddle attachment or a hand mixer, mix on medium speed for about 3 to 5 minutes until the dough is smooth and slightly sticky.
Grease a 9×5-inch loaf pan with butter or non-stick spray. Transfer the dough into the pan and smooth the top with a wet spatula or wet hands.
Cover the loaf pan loosely with a clean kitchen towel or plastic wrap and let the dough rise in a warm place for 30 to 40 minutes, or until it has risen to about 1 inch above the rim of the pan.
Preheat the oven to 375°F (190°C).
Bake the bread for 40 to 45 minutes until the top is golden brown and a toothpick inserted in the center comes out clean.
Remove the bread from the oven and let it cool in the pan for 10 minutes.
Transfer the bread to a wire rack and cool completely before slicing.