Guanylyltransferase that catalyzes the activation of phosphoenolpyruvate (PEP) as enolpyruvoyl-2-diphospho-5'-guanosine, via the condensation of PEP with GTP. It is involved in the biosynthesis of coenzyme F420, a hydride carrier cofactor. GTP + H(+) + phosphoenolpyruvate = diphosphate + enolpyruvoyl-2-diphospho-5'-guanosine Cofactor biosynthesis; coenzyme F420 biosynthesis. Cells lacking this gene accumulate 7,8-didemethyl-8-hydroxy-5-deazariboflavin (Fo) to five times the wild-type levels but do not produce F420. They are also unable to decolorize triphenylmethane dye, and are sensitive to oxidative stress caused by the redox-cycling agents plumbagin and menadione. Belongs to the CofC family.