Required for polymorphic O-glycosylation of GspB, a serine-rich repeat cell wall protein encoded upstream in the same operon. A substrate-binding protein that is part of the accessory SecA2/SecY2 system specifically required to export GspB. The GtfA-GtfB complex adds GlcNAc from UDP-GlcNAc to GspB, attaching the first sugar residue. Upon coexpression in E.coli with GtfA glycosylates GspB constructs (PubMed:15489421). Binds the GspB protein substrate; alone this subunit only recognizes partially glycosylated GspB, but is constrained by GtfA to also recognize unglycosylated protein. The enzyme probably modifies its tertiary conformation by opening and closing its intersubunit interfaces to accomodate the increasingly glycosylated substrate (Probable). Protein modification; protein glycosylation. Forms a heterotetramer with 2 subunits each of GtfA and GtfB (PubMed:26884191). Part of the accessory SecA2/SecY2 protein translocation apparatus required to export cell wall protein GspB (Probable). Loss of export of cell wall protein GspB; transcription should not be affected. Non-glycosylated GspB may form aggregates. Belongs to the GtfB family.