Catalyzes the reversible NADP-dependent oxidation of gluconate to 5-ketogluconate. Is involved in the non-phosphorylative, ketogenic oxidation of glucose. Is almost inactive with NAD as cosubstrate. Displays high substrate specificity since D-Glucose, D-sorbitol, and D-mannitol are not oxidized by the enzyme, and 2-ketogluconate and L-sorbose are not reduced. Can accept D-fructose as a substrate, with a rate that is only 10% of the rate of 5-ketogluconate reduction. D-gluconate + NADP(+) = 5-dehydro-D-gluconate + H(+) + NADPH Optimum pH is 10.0 for gluconate oxidation. Optimum pH is 5.0 for 5-ketogluconate reduction. Homodimer. Belongs to the short-chain dehydrogenases/reductases (SDR) family.