Involved in the biosynthesis of mycobacterial methylglucose lipopolysaccharides (MGLP). Catalyzes the dephosphorylation of glucosyl-3-phosphoglycerate (GPG) to glucosylglycerate. (2R)-2-O-(alpha-D-glucopyranosyl)-3-phospho-glycerate + H2O = (2R)-2-O-(alpha-D-glucopyranosyl)-glycerate + phosphate Homodimer. Belongs to the phosphoglycerate mutase family.