A gluten-free diet is a diet that excludes the protein gluten. Gluten is found in grains such as wheat, barley, rye, and a cross between wheat and rye called triticale.A gluten-free diet is primarily used to treat celiac disease. Gluten causes inflammation in the small intestines of people with celiac disease.efinition. A gluten-free diet is a diet that excludes the protein gluten. Gluten is found in grains such as wheat, barley, rye, and a cross between wheat and rye called triticale. A gluten-free diet is primarily used to treat celiac disease.