Involved in the degradation of glucose via the Entner-Doudoroff pathway. Catalyzes the dehydration of gluconate to produce 2-keto-3-deoxygluconate (KDG). It is not able to use D-galactonate as substrate. D-gluconate = 2-dehydro-3-deoxy-D-gluconate + H2O Binds 1 Mg(2+) ion per subunit. Carbohydrate acid metabolism; D-gluconate degradation. Belongs to the mandelate racemase/muconate lactonizing enzyme family. GaD subfamily.