To make this Spinach Artichoke Grilled Cheese Recipe gluten-free, replace the sourdough bread with certified gluten-free bread. Verify that the cream cheese, mozzarella, Parmesan, butter, garlic powder, red pepper flakes, canned or jarred artichoke hearts, and all seasonings are gluten-free. If using frozen spinach, confirm that it contains no gluten-based additives. Use a clean skillet, utensils, and prep surface to prevent cross-contamination.