If it's only one tablespoon you need, definitely use normal plain flour or potato flour. It wouldn't be worth your while to go out and buy a GF plain flour mix for only that small quantity- like many GF products, GF flour mixes are hideously expensive compared to normal wheat based products! We coeliacs sure pay a premium for gluten free food items! (just had to have a whine today to food manufacturers- some of us actually have to have a gluten free lifestyle to stay healthy and not get complications from eating gluten, not because we' wish to be fashionable!!!!)