Guanylyltransferase that catalyzes the synthesis of adenosylcobinamide-GDP (AdoCbi-GDP) from adenosylcobinamide-phosphate (AdoCbi-P) and GTP. Is involved in adenosylcobalamin biosynthesis. Binds one GTP per dimer. Cannot use other NTPs or GDP. Does not display AdoCbi kinase activity. Is also able to catalyze the condensation of 2-phospho-L-lactate (LP) with GTP in vitro to form PPi and (2S)-lactyl-2-diphospho-5'-guanosine (LPPG), but is much less efficient than CofC, the presumed enzyme catalyzing this reaction in vivo. adenosylcob(III)inamide phosphate + GTP + H(+) = adenosylcob(III)inamide-GDP + diphosphate Cofactor biosynthesis; adenosylcobalamin biosynthesis. Homodimer. Binds GTP first, before binding AdoCbi-P.