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). To a lesser extent can also use GDP-Glc but not UDP-Gal or UDP-GlcNAc as the sugar donor. (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 (2R)-3-phosphoglycerate + GDP-D-glucose = (2R)-2-O-(alpha-D-glucopyranosyl)-3-phospho-glycerate + GDP + H(+) Homodimer. Belongs to the glycosyltransferase 2 family.