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. Belongs to the CofC family. Extended N-terminus.