Catalyzes the isomerization-deamination of galactosamine 6-phosphate to form tagatofuranose 6-phosphate and ammonium ion. D-galactosamine 6-phosphate + H2O = D-tagatopyranose 1-phosphate + NH4(+) In contrast to E.coli strains C and EC3132, K-12 strains cannot grow on N-acetylgalactosamine and D-galactosamine, because they carry a deletion and thus lack active PTS systems specific for these compounds. Therefore, AgaS in K-12 strains is not involved in the degradation of these compounds. Belongs to the SIS family. AgaS subfamily.