Required for polymorphic O-glycosylation of the serine-rich repeat protein in this bacteria. Catalyzes the second step in glycosylation by transferring glucose from UDP-glucose to the terminal GlcNAc moiety of the 3-O-(N-acetyl-alpha-D-glucosaminyl)-L-seryl-[protein] resulting from the first glycosylation step. Part of the accessory SecA2/SecY2 system specifically required to export GspB, a serine-rich repeat cell wall protein encoded upstream in the same operon. Protein modification; protein glycosylation. Homotetramer; a dimer of dimers. Dimerizes via the C-terminus; dimerization is required for tetramer formation. Binds protein substrate via an exposed loop in the N-terminus. Alters carbohydrate composition of cell wall protein GspB, about 20% reduction in platelet binding by whole cells. Belongs to the Gtf3 glucosyltransferase family.