Involved in the biosynthesis of 6-O-methylglucose lipopolysaccarides (MGLPs). Catalyzes the transfer of a glucose (Glc) moiety from uridine diphosphate (UDP-Glc) to the position 2 of 3-phospho-D-glycerate (3-PGA) to form glucosyl-3-phosphoglycerate (GPG). GpgS is most active with UDP-glucose, followed by GDP-glucose, ADP-glucose, and to a lesser extent, TDP-glucose. 3-PGA is the only acceptor for these glucosyl donors. (2R)-3-phosphoglycerate + an NDP-alpha-D-glucose = (2R)-2-O-(alpha-D-glucopyranosyl)-3-phospho-glycerate + a ribonucleoside 5'-diphosphate + H(+) Requires divalent cations for activity in the following order of efficiency: Mn(2+), Co(2+), Mg(2+) and Ni(2+) ions. Optimum pH is 8. Optimum temperature is 90 degrees Celsius. GpgS is inactive at 40 degrees Celsius, but the activity increases dramatically above 50 degrees Celsius. At 100 degrees Celsius, GpgS retains 33% of the total activity. Homodimer. Belongs to the glycosyltransferase 2 family.