More accurately, all liquor is MOSTLY gluten free.All liquor is gluten free. The protein that triggers celiac or gluten reactions is too heavy to pass through the distillation process. It stays behind in the wash instead of vaporizing with the alcohol.
The limit for ‘Gluten Free’ labeling is 20 or less parts per million. So, like non-alcohol beer, or Fat Free food, there can be a minute amount. Depending on each person’s sensitivity that little may or may not trigger a reaction.
What causes the intolerance reaction in most people is caramel coloring that’s been added to spirits. Coloring is allowed in Scotch, some Tequila - particularly mixto; Rum and flavored whiskies. It is not allowed in Bourbon or Tennessee Whiskey.