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. Can also use (2S)-2-phospholactate (2-PL), with lower turnover, and has weak activity with phosphoenolpyruvate (PEP). (2R)-3-phosphoglycerate + GTP + H(+) = 3-[(R)-glyceryl]-diphospho-5'-guanosine + diphosphate (2S)-2-phospholactate + GTP + H(+) = (2S)-lactyl-2-diphospho-5'-guanosine + diphosphate Cofactor biosynthesis; coenzyme F420 biosynthesis. Belongs to the CofC family.