Condiments are where gluten hides most reliably. Soy sauce is a wheat product, most Worcestershire outside the US contains malt vinegar, gravy and roux are wheat-thickened by definition, and bouillon is one of the most common sources of hidden barley.
A meal built entirely from naturally gluten-free ingredients is routinely undone at the last step by a spoonful of something from a jar.

Malt vinegar is the exception worth knowing, because vinegar is otherwise a safe category. Distilled white vinegar is gluten free regardless of what it was distilled from. Malt vinegar is not distilled, so the barley protein remains. UK and Commonwealth Worcestershire sauce is built on malt vinegar, while the US formulation is built on distilled vinegar and is generally gluten free.
Bouillon is the quiet one. A stock cube is a flavour concentrate, and hydrolyzed wheat protein and barley-derived yeast extract are both standard tools for building savoury depth cheaply. Soups, risottos, braises and pan sauces inherit whatever was in the cube.
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Sorted by what the answer actually is, rather than alphabetically — because the useful question in a shop is “which pile is this in.”
Brewed with wheat. See the soy sauce guide.
Brewed from barley and not distilled, so the protein remains.
Wheat flour is the structural thickener.
US formulations use distilled vinegar and are usually fine; UK and Commonwealth versions use malt vinegar.
A very common source of hidden barley and hydrolyzed wheat protein.
Usually fine, but soy sauce and malt appear in Asian-style and some creamy dressings.
Soy sauce and malt vinegar both turn up; beer appears in some.
Distillation removes gluten protein regardless of the starting grain.
Generally fine. Some specialty mustards use beer or malt.
Chili, vinegar, garlic, salt.
It depends on the country, and the same brand can give different answers on different continents. The US formulation is built on distilled white vinegar and is generally gluten free. UK and Commonwealth formulations use malt vinegar, which comes from barley and is not distilled.
Distilled vinegar is, regardless of what it was distilled from, because gluten protein does not carry over in distillation. Malt vinegar is the exception: it is brewed from barley and not distilled, so it retains gluten.
Traditional gravy is not. Wheat flour is the thickener. Gravy thickened with cornstarch or a gluten-free blend is fine, and gluten-free gravy granules exist. Restaurant gravy should be assumed to be flour-thickened unless stated otherwise.
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