McDonald's smoothies are gluten free.
A quick check of McDonald's online nutrition tools (which can be found online after a quick Google search) reveals that the ingredients for McDonald's smoothie are gluten free.
Here is the ingredients list for one of McDonald's Strawberry Banana Smoothie:
Strawberry Banana Fruit Blend (Strawberry puree, banana puree, water, sugar, concentrated apple juice, contains less than 1% of the following: Cellulose powder, natural (botanical source) and artificial flavors, xanthan gum, citric acid, colored with fruit and vegetable juice, pectin, ascorbic acid (preservative)), Low Fat Yogurt (Cultured Grade A reduced fat milk, sugar, whey protein concentrate, fructose, corn starch, gelatin. Contains active yogurt cultures), Ice (Ice).
You can substitute this recipe for any of the other varieties of McDonald's smoothie, obviously swapping out the fruit ingredient for the relevant one for the flavor you wish to know about.
