Flour thickens gumbo when cooks make a roux, combining it with fat and cooking the mixture before adding liquid.
A roux is a cooked mixture of flour and fat. In Louisiana cooking, the fat may be butter, vegetable oil, or animal fat, while the flour gradually darkens as it cooks. Gumbo commonly uses a brown or dark roux, whose toasted flavor is as important as its thickening ability.
The longer a roux cooks, the darker and more flavorful it becomes, but dark roux has less thickening power than pale roux. This creates a practical trade-off between flavor and viscosity. Stirring is important because flour can burn quickly.
Gumbo recipes differ by region and family. Some also use okra or filé powder as thickeners, but those ingredients do not replace the basic flour-and-fat roux in every style. Gumbo may contain seafood, chicken, sausage, or combinations of these.