Preheat the oven to 400°F and spray a 12-cup muffin pan with gluten-free nonstick spray and line with paper liners.
In a large bowl, whisk together the flour, xanthan gum, oats, granulated sugar, brown sugar, baking powder, baking soda, salt, and cinnamon. Break up any big brown sugar clumps as necessary.
In a medium bowl, beat the eggs. Whisk in the mashed banana, vanilla, oil, and milk. Pour the wet ingredients into the dry ingredients and mix well until combined.
Divide the batter between the 12 muffin cups and set aside while you make the topping. The batter needs to sit at least 10-15 minutes before baking.
To make the streusel topping, mix together the melted butter, brown sugar, flour, xanthan gum, and oats in a small bowl. Mix until all ingredients are moistened. Sprinkle the topping over the muffins, dividing evenly between all 12.
Bake for 18-22 minutes or until a toothpick comes out clean. Cool on a wire rack. Store leftover muffins in an airtight container for up to 3 days.