Involved in the pathway of N-acetyl-D-galactosamine degradation. Catalyzes the conversion of D-galactosamine 6-phosphate (GalN-6-P) to D-tagatofuranose 6-phosphate (Tag-6-P). It can also catalyze the conversion of D-glucosamine 6-phosphate. D-galactosamine 6-phosphate + H2O = D-tagatopyranose 1-phosphate + NH4(+) alpha-D-glucosamine 6-phosphate + H2O = beta-D-fructose 6-phosphate + NH4(+) In Shewanella sp., the active phosphotransferase system (PTS) specific for the transport of GalNAc and GalN is replaced by a set of GalNAc- and GalN-specific permeases and kinases (AgaP and AgaK, respectively). Belongs to the SIS family. AgaS subfamily.