To make this Cheesy Italian Sausage and Peppers Skillet gluten-free, use certified gluten-free Italian sausage, as some sausage brands may contain fillers or seasonings with gluten. Also confirm that the mozzarella, cheddar cheese, olive oil, and nonstick cooking spray are gluten-free. Serve with gluten-free sides such as rice, mashed potatoes, roasted vegetables, salad, or certified gluten-free bread.