US FDA Food Labeling Regulations (21 CFR Part 101) 101.91: Gluten-free labeling: 'gluten-free', 'no gluten', 'free of gluten' and 'without gluten' only on foods containing less than 20 ppm gluten with no gluten-containing grain or derived ingredient from which gluten was not stripped out, including fermented and hydrolyzed foods and the records they require (101.91)

What does 101.91 require?

A food may be labeled gluten-free only if it does not contain an ingredient that is a gluten-containing grain (wheat, rye, barley and their crossbred hybrids), an ingredient derived from such a grain from which gluten was not stripped out, or such an ingredient processed to remove gluten if its use leaves the food at or above 20 ppm gluten, and the food in any case contains less than 20 ppm gluten. Foods inherently gluten-free may use the claim provided the label does not imply other similar foods contain gluten unless true. For fermented or hydrolyzed foods and ingredients, where gluten cannot be reliably measured, the manufacturer must make and keep records showing the food meets the requirements before fermentation or hydrolysis, that the food was evaluated for potential gluten cross-contact, and that any cross-contact was adequately controlled. Distilled foods are evaluated by the absence of protein.