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. (2S)-2-phospholactate + GTP + H(+) = (2S)-lactyl-2-diphospho-5'-guanosine + diphosphate Cofactor biosynthesis; coenzyme F420 biosynthesis. Homodimer. Belongs to the CofC family.