Chelsea icing sugar (powdered sugar) has always been gluten free but I notice the new packaging clearly labels it as gluten free. Great idea.
Be careful with brands of icing sugar, as starch is often added to the powdered sugar to keep it free from lumps. Some brands are mixed with tapioca starch and some icing sugars are mixed with wheat starch.
Always check the ingredients if it is not labelled as gluten free.