Part of the heterotrimer that catalyzes the conversion of D-gluconate to 2-dehydro-D-gluconate. This subunit functions as the dehydrogenase. A + D-gluconate = 2-dehydro-D-gluconate + AH2 Optimum pH is 5.0. Optimum temperature is 30 degrees Celsius. Heterotrimer. Belongs to the GMC oxidoreductase family.