Required for polymorphic O-glycosylation of the serine-rich repeat protein (SRRP) in this bacteria. A stabilizing protein that is part of the accessory SecA2/SecY2 system specifically required to export serine-rich repeat cell wall proteins encoded in the same operon. The GtfA-GtfB complex adds GlcNAc from UDP-GlcNAc to the substrate protein, attaching the first sugar residue. Stabilizes the glycosylation activity of GtfA. Has no N-acetylglucosaminyl transferase activity on its own. Protein modification; protein glycosylation. Forms a heterotetramer with 2 subunits each of GtfA and GtfB. Part of the accessory SecA2/SecY2 protein translocation apparatus. Belongs to the GtfB family.