Catalyzes the dehydration of D-galactonate to 2-keto-3-deoxy-D-galactonate. D-galactonate = 2-dehydro-3-deoxy-D-galactonate + H2O Binds 1 Mg(2+) ion per subunit. Carbohydrate acid metabolism; D-galactonate degradation; D-glyceraldehyde 3-phosphate and pyruvate from D-galactonate: step 1/3. Reaction proceeds via an anti dehydration. Belongs to the mandelate racemase/muconate lactonizing enzyme family. GalD subfamily.