Involved in the biosynthesis of the compatible solute mannosylglucosylglycerate through a nonphosphorylating pathway. Catalyzes the synthesis of mannosylglucosylglycerate (MGG) from glucosylglycerate (GG) and GDP-mannose. (2R)-2-O-(alpha-D-glucopyranosyl)-glycerate + GDP-alpha-D-mannose = (2R)-2-O-[alpha-D-mannopyranosyl-(1->2)-alpha-D-glucopyranosyl]-glycerate + GDP + H(+) Belongs to the glycosyltransferase group 1 family.