Involved in the biosynthesis of the compatible solute alpha-D-mannosyl-glycerate (MG) (PubMed:23179444). Catalyzes the condensation of GDP-alpha-D-mannose (GDP-Man) with D-glycerate to produce alpha-D-mannosyl-glycerate (PubMed:23179444). Can also use GDP-alpha-D-glucose (GDP-Glc) as sugar donor to produce alpha-D-glucopyranosyl-glycerate (GG) (PubMed:23179444). (R)-glycerate + GDP-alpha-D-mannose = (2R)-2-O-(alpha-D-mannosyl)-glycerate + GDP + H(+) (R)-glycerate + GDP-alpha-D-glucose = (2R)-2-O-(alpha-D-glucopyranosyl)-glycerate + GDP + H(+) Activity is not dependent on divalent cations, but it is enhanced by Mg(2+). Optimum pH is 8.0. Optimum temperature is 50 degrees Celsius. Belongs to the glycosyltransferase 78 family.