Involved in the degradation of 3,6-anhydro-L-galactose, which is the major monomeric sugar of red macroalgae. Catalyzes the third step of the pathway, the NAD(+)-dependent oxidation of 2-dehydro-3-deoxy-L-galactonate (L-KDGal) to 3-deoxy-D-glycero-2,5-hexodiulosonate (L-DDGal). 2-dehydro-3-deoxy-L-galactonate + NAD(+) = 3-deoxy-D-glycero-2,5-hexodiulosonate + H(+) + NADH Binds 2 Zn(2+) ions per subunit. Belongs to the zinc-containing alcohol dehydrogenase family.