Guanylyltransferase that catalyzes the activation of (2R)-3-phosphoglycerate (3PG) as 3-[(R)-glyceryl]-diphospho-5'-guanosine, via the condensation of 3PG with GTP. It is involved in the biosynthesis of a derivative of the hydride carrier cofactor coenzyme F420, 3PG-F420. (2R)-3-phosphoglycerate + GTP + H(+) = 3-[(R)-glyceryl]-diphospho-5'-guanosine + diphosphate Cofactor biosynthesis; coenzyme F420 biosynthesis. Belongs to the CofC family.