Is responsible for transferring galactose-1-phosphate to the lipid precursor undecaprenol phosphate in the first steps of O-polysaccharide biosynthesis. di-trans,octa-cis-undecaprenyl phosphate + UDP-alpha-D-galactose = alpha-D-galactosyl-di-trans,octa-cis-undecaprenyl diphosphate + UMP Bacterial outer membrane biogenesis; LPS O-antigen biosynthesis. Belongs to the bacterial sugar transferase family.