Catalyzes the addition of Gal, the second sugar moiety of the O7-antigen repeating unit, to GlcNAc-pyrophosphate-undecaprenol. N-acetyl-alpha-D-glucosaminyl-di-trans,octa-cis-undecaprenyl diphosphate + UDP-alpha-D-galactose = beta-D-Gal-(1->3)-alpha-D-GlcNAc-di-trans,octa-cis-undecaprenyl diphosphate + H(+) + UDP Mn(2+) ion. Can also use other divalent metal cations. Optimum pH is 7. Bacterial outer membrane biogenesis; LPS O-antigen biosynthesis. Belongs to the glycosyltransferase 2 family.