Involved in the biosynthesis of 6-O-methylglucose lipopolysaccarides (MGLPs). Catalyzes the transfer of the glucose moiety from UDP-alpha-D-glucose (UDP-Glc) to the position 2 of 3-phospho-D-glycerate (3-PGA) to form glucosyl-3-phosphoglycerate (GPG). (2R)-3-phosphoglycerate + an NDP-alpha-D-glucose = (2R)-2-O-(alpha-D-glucopyranosyl)-3-phospho-glycerate + a ribonucleoside 5'-diphosphate + H(+) (2R)-3-phosphoglycerate + UDP-alpha-D-glucose = (2R)-2-O-(alpha-D-glucopyranosyl)-3-phospho-glycerate + H(+) + UDP Requires divalent cations for activity. Mg(2+) gives the maximal activity, but the enzyme can also use Ca(2+), Mn(2+), Fe(2+) or Co(2+) ions, but not Zn(2+) or Cu(2+) ions. Homodimer. Was identified as a high-confidence drug target. The glucosyltransferase GpgS does not possess a putative nucleophile residue that could result in the formation of a glycosyl-enzyme covalent intermediate. Therefore, it is expected that the reaction would proceed through a front-side substrate-assisted SNi-type mechanism where the breaking of the glucosidic bond is stabilized by the interaction of the acceptor hydrogen atom O3 of the acceptor 3-PGA with the beta-phosphate of the nucleotide sugar. Belongs to the glycosyltransferase 2 family.