Involved in the biosynthesis of 6-O-methylglucose lipopolysaccarides (MGLPs). Catalyzes the transfer of the glucose moiety from a nuleotide sugar such as UDP-alpha-D-glucose to the position 2 of 3-phospho-D-glycerate (3-PGA) to form glucosyl-3-phosphoglycerate (GPG). It can use UDP-glucose, ADP-glucose and GDP-glucose as sugar donor substrates with decreasing affinity and with 3-PGA as an acceptor. D-glycerate can only be an acceptor with ADP-glucose and at a very low rate