You don’t need to give up a thicker sauce because you’re gluten-free. To make this sauce gluten-free, omit the flour. You can leave it like that which results in a much thinner sauce or you can make it thicker. To make it thicker and still gluten-free, replace the flour with cornstarch slurry. Cornstarch slurry consists of equal parts cold water and cornstarch. Just add it to the sauce when it’s simmering.