Transfers a phosphoglycerol residue from phosphatidylglycerol to the membrane-bound nascent glucan backbones. phosphatidylglycerol + membrane-derived-oligosaccharide D-glucose = 1,2-diacyl-sn-glycerol + membrane-derived-oligosaccharide 6-(glycerophospho)-D-glucose. Glycan metabolism; osmoregulated periplasmic glucan (OPG) biosynthesis. Belongs to the OpgB family. Truncated N-terminus. Truncated N-terminus.