Involved in galactitol and D-altritol catabolism. Catalyzes the epimerization of D-tagatose 6-phosphate to D-fructose 6-phosphate. keto-D-tagatose 6-phosphate = keto-D-fructose 6-phosphate kcat is 8.7 sec(-1). Carbohydrate metabolism. Up-regulated by growth on D-altritol or galactitol. Deletion of the gene leads to slow-growth phenotypes for both D-galactitol and D-altritol. Belongs to the GatZ/KbaZ family.