mRNA capping. Transfers a GMP cap onto the end of mRNA that terminates with a 5'-diphosphate tail. a 5'-end diphospho-ribonucleoside in mRNA + GTP + H(+) = a 5'-end (5'-triphosphoguanosine)-(ribonucleoside) in mRNA + diphosphate Monomer. Belongs to the eukaryotic GTase family.