To make this recipe gluten free, use a certified gluten-free rye-style bread or another sturdy gluten-free sandwich bread. Also verify that the corned beef, sauerkraut, horseradish, pickle relish, ketchup, mayonnaise, sour cream, and Swiss cheese are labeled gluten free, since some deli meats and condiments can contain additives or be subject to cross-contamination. Because gluten-free bread can brown faster and be more delicate, cook the sandwich over slightly lower heat and flip carefully to prevent breakage.