To make these Coffee Cake Muffins gluten-free, replace the all-purpose flour in both the muffin batter and streusel topping with a certified gluten-free 1:1 baking flour blend. Confirm that the baking powder, baking soda, confectioners’ sugar, brown sugar, granulated sugar, cinnamon, vanilla extract, sour cream, milk, butter, eggs, and salt are labeled gluten-free. Use clean mixing bowls, measuring tools, muffin pans, liners, spatulas, and cooling racks to prevent gluten contamination.