Involved in the biosynthesis of the compatible solute mannosylglucosylglycerate through a phosphorylating pathway. Catalyzes the conversion of glucosyl-3-phosphoglycerate (GPG) to mannosylglucosyl-3-phosphoglycerate (MGPG). (2R)-2-O-(alpha-D-glucopyranosyl)-3-phospho-glycerate + GDP-alpha-D-mannose = (2R)-2-O-[alpha-D-mannopyranosyl-(1->2)-alpha-D-glucopyranosyl]-3-phospho-glycerate + GDP + H(+) Not strictly dependent on divalent cations, but the presence of Mn(2+), Ca(2+), Mg(2+) or Co(2+) stimulates activity. Optimum pH is 9.0 (at 60 degrees Celsius). Optimum temperature is 90 degrees Celsius. Monomer in solution.