Involved in the biosynthesis of mycobacterial methylglucose lipopolysaccharides (MGLPs). Catalyzes the dephosphorylation of glucosyl-3-phosphoglycerate (GPG) to glucosylglycerate (GG). GPG is the preferred substrate, but GpgP also exhibits low dephosphorylation activity on mannosyl-3-phosphoglycerate (MPG) and mannosylglucosyl-3-phosphoglycerate (MGPG) in vitro. Shows only trace of phosphoglycerate mutase (PGM) activity. (2R)-2-O-(alpha-D-glucopyranosyl)-3-phospho-glycerate + H2O = (2R)-2-O-(alpha-D-glucopyranosyl)-glycerate + phosphate 2-O-(alpha-D-mannosyl)-3-phosphoglycerate + H2O = (2R)-2-O-(alpha-D-mannosyl)-glycerate + phosphate (2R)-2-O-[alpha-D-mannopyranosyl-(1->2)-alpha-D-glucopyranosyl]-3-phospho-glycerate + H2O = (2R)-2-O-[alpha-D-mannopyranosyl-(1->2)-alpha-D-glucopyranosyl]-glycerate + phosphate Progressively inhibited by cobalt ions at concentrations between 10-50 mM and by copper ions at any concentration between 1-50 mM. Optimum pH is 7 (at 37 degrees Celsius). Is active between pH 4.0 and 10.0. Optimum temperature is 45 degrees Celsius. Is active between 20 and 50 degrees Celsius. Homodimer (PubMed:22355692). Dimerization of the enzyme is essential for its dephosphorylation activity (PubMed:24914210). Belongs to the phosphoglycerate mutase family.