It depends
Wheat flour is the traditional thickener for gravy and a common carrier in bouillon cubes and soup bases, and barley malt or yeast extract often supplies the savoury note — none of which a US label must disclose.
Concentrated stock, sold as a cube, a powder, a paste or a carton. Salt and flavouring do most of the work; the rest is some combination of yeast extract, hydrolysed protein, fat and a thickener.
Because it is the most efficient way there is to turn a safe meal into an unsafe one. Nobody eats bouillon on its own — it goes into the soup, the risotto, the gravy, the braise, the stuffing, the pan sauce — so one wrong cube contaminates a whole dish that contained nothing else of concern. It also stacks several ambiguous ingredients at once: wheat flour as a thickener, yeast extract that may be spent brewer's yeast, hydrolysed protein of unstated origin.
Buy one certified gluten-free stock and make it the house default. It is a cheap swap that removes a whole class of accidents at once, and it is more reliable than re-checking each product each time — bouillon reformulates often, and the brand you cleared last year is not necessarily the one on the shelf today.
Because the answer depends on the rest of the label, scanning the actual product is the only way to resolve it — CiCi checks for a gluten-free claim, the allergen statement, and which agency regulates that label, then tells you which way it lands.
Every one of these is the same ingredient. The number of names is a large part of why gluten slips past people who read every label.
Point CiCi at the barcode on the product you're holding and get this same reasoning in about two seconds, plus the carbs and the full nutrition panel. 360,000+ packaged foods are already read and waiting.
The aisle guides below cover Bouillon in context — which is usually the more useful question, because an ingredient rarely appears alone.
This answer comes from CiCi's ingredient lexicon — the same rules the scanner uses, not a separate article. Where reputable sources disagree, CiCi takes the conservative position and says so.