No, this recipe is not gluten free because we ask for flour as a thickener. In case you want to keep the stew gluten-free but still have a thick stew sauce, leave the crockpot uncovered for the last 2 hours, so steam can be released and it will naturally reduce and thicken. You can thicken it also it with corn starch instead of white flour to keep it gluten free.
PLEASE NOTE: For Celiacs and High Gluten sensitivity this recipe will still not be gluten free safe by changing the thickening methods. Guinness is considered a gluten-filled beer because it uses malted barley.
If you are simply trying to reduce your processed flour intake and carbs then changing the thickener will work, but for allergies, this recipe would need to use a certified gluten free beer and not Guinness.
So while yes, the soup would be gluten free with Guinness,  and safe for regular diets and consumption, it is not gluten free to the allergen standards of those with high sensativity.