Guanylyltransferase that catalyzes the activation of (2S)-2-phospholactate (2-PL) as (2S)-lactyl-2-diphospho-5'-guanosine, via the condensation of 2-PL with GTP. It is involved in the biosynthesis of coenzyme F420, a hydride carrier cofactor (PubMed:18260642, PubMed:31469543). Is able to utilize other purine nucleotides including ATP, dGTP and ITP as cosubstrates in place of GTP but with a lower activity. Does not display lactate kinase activity (PubMed:18260642). (2S)-2-phospholactate + GTP + H(+) = (2S)-lactyl-2-diphospho-5'-guanosine + diphosphate Retains all activity when heated up to 90 degrees Celsius for 10 minutes and retained 38% activity after 10 minutes at 100 degrees Celsius. Cofactor biosynthesis; coenzyme F420 biosynthesis. Homodimer. Belongs to the CofC family.