Plain brandy usually is made from grapes, and (similar to wine) should be gluten-free down to a pretty low level. So if you're following the gluten-free diet because you have celiac disease or gluten sensitivity, plain brandy should be safe for you.
Pure grape-based brandy types include cognac, armagnac and pisco (a Spanish brandy).
However, the gluten-free status of brandy gets a little more confusing when it comes to fruit-based brandy and flavored brandy, since it's possible for the flavorings to contain gluten (usually here the culprit is a wheat-based natural flavor or a barley-based sweetener).
Unlike food manufacturers, the makers of alcoholic beverages do not need to list their ingredients, and many flavored brandy manufacturers keep their formulas a closely-guarded secret.
Calvados Considered Gluten-Free, But Ouzo Questionable
Calvados, an apple-based brandy, is gluten-free, as is eau-de-vie (French fruit brandy). Slivovitz, a brandy made of plums from Eastern Europe, also should be gluten-free (but as with any food or drink that doesn't list its ingredients, I'd advise moving cautiously).
Ouzo, a traditional Greek drink that contains anise and spices, also is considered brandy by some connoisseurs. Greek law requires ouzo to contain at least 30% grapes; the remainder can be other forms of fruit, potatoes or even grains, including gluten grains (distilled gluten grains are considered to be gluten-free, but many of us do react; learn more in Is Alcohol Gluten-Free?).
The flavorings in ouzo can contain crushed grains (yes, including gluten grains), as well, although those flavorings usually made up mainly of spices. If you're in doubt about the gluten-free status of a particular ouzo, contact the manufacturer (if possible) or stick with a more reliably gluten-free cocktail.
One potential option for ouzo lovers: Americanaki Ouzo, made by Old Sugar Distillery in Madison, Wis., is crafted from beet sugar and other gluten-free ingredients in a facility that does not use gluten ingredients, according to owner and distiller Nathan Greenawalt.
Oak Barrel Flour Seal A Gluten Problem?
Most brandies are aged in oak barrels or casks, which brings up one small potential source of gluten cross-contamination traditionally, oak casks used to ferment wine and brandy are sealed with a wheat or rye flour paste.
Any gluten from this seal that remains in the final product is definitely going to be too low to detect with current gluten testing technology it's likely well below 1 part per million (for comparison, 20 parts per million of gluten is considered "gluten-free," although many people react to lower levels than that). However, if you try a brandy and believe you react to it, this flour-based cask seal is one possible reason, especially if you tend to be particularly sensitive.
Read more on brandy from About.com's Guide to Cocktails: