Powdered sugar, also known as confectioners sugar, is gluten-free. The only ingredients in powdered sugar are cornstarch and sugar that has been finely ground.
The ratio of cornstarch to sugar is 3 percent cornstarch to 97 percent sugar. The starch is used only to keep the sugar from clumping together. Since gluten is a wheat product, pure cornstarch contains no gluten in theory. However, care should be taken for sufferers of celiac disease for whom even a tiny amount of gluten can trigger symptoms. Factories are not always diligent about keeping trace amounts of other products they manufacture from cross-contaminating their foods.