Catalyzes the reduction of 2,3-diketo-L-gulonate in the presence of NADH, to form 3-keto-L-gulonate. 3-dehydro-L-gulonate + NAD(+) = 2,3-dioxo-L-gulonate + H(+) + NADH 3-dehydro-L-gulonate + NADP(+) = 2,3-dioxo-L-gulonate + H(+) + NADPH Homodimer. Belongs to the LDH2/MDH2 oxidoreductase family. DlgD subfamily.