Transfers N-acetyl-galactosamine (GalNAc) to undecaprenyl phosphate, a step in the assembly of the repeating-unit of the O-antigen. Shows no activity with UDP-N-acetyl-alpha-D-glucosamine. di-trans,octa-cis-undecaprenyl phosphate + UDP-N-acetyl-alpha-D-galactosamine = N-acetyl-alpha-D-galactosaminyl-di-trans,octa-cis-undecaprenyl diphosphate + UMP Bacterial outer membrane biogenesis; LPS O-antigen biosynthesis. Belongs to the bacterial sugar transferase family.