Catalyzes the oxidative decarboxylation of 6-phosphogluconate to ribulose 5-phosphate and CO(2), with concomitant reduction of NAD to NADH. Does not contribute to oxidative pentose phosphate (PP) pathway fluxes during growth on glucose. The functional role of GntZ remains obscure. 6-phospho-D-gluconate + NAD(+) = CO2 + D-ribulose 5-phosphate + NADH Homodimer. Cells lacking this gene exhibit no detectable phenotype on glucose as the sole carbon source, and they grow normally on gluconate. Belongs to the 6-phosphogluconate dehydrogenase family.