To make these Snickerdoodles gluten-free, replace the all-purpose flour with a certified gluten-free 1:1 baking flour blend suitable for cookies. Confirm that the cream of tartar, baking soda, salt, butter, eggs, sugar, and ground cinnamon are labeled gluten-free and free from cross-contact warnings. Some spices and baking ingredients may be processed on shared equipment, so check labels carefully. Use clean mixing bowls, baking sheets, parchment paper, measuring tools, spatulas, and cooling racks to prevent gluten contamination.