To make it gluten free, replace the Guinness with a gluten free stout beer and use a gluten free Worchestershire sauce. In addition to this, leave out the flour and thicken at the end by stirring in a cornstarch slurry and bringing to a boil to thicken the broth into more of a gravy. A slurry is 1 Tbsp. cornstarch (or arrowroot) mixed with 2 Tbsp. cold water.