Catalyzes the reduction of 2-keto-D-gluconate to gluconate. Can also catalyze the reduction of 2-keto-L-gulonate. Can use both NADH and NADPH efficiently, with a slight preference for NADPH. D-gluconate + NADP(+) = 2-dehydro-D-gluconate + H(+) + NADPH kcat is 48.2 sec(-1). Homohexamer. Belongs to the D-isomer specific 2-hydroxyacid dehydrogenase family.