Second step of mRNA capping. Transfer of the GMP moiety of GTP to the 5'-end of RNA via an enzyme-GMP covalent reaction intermediate. a 5'-end diphospho-ribonucleoside in mRNA + GTP + H(+) = a 5'-end (5'-triphosphoguanosine)-(ribonucleoside) in mRNA + diphosphate Heterodimer. The mRNA-capping enzyme is composed of two separate chains alpha and beta, respectively a mRNA guanylyltransferase and an mRNA 5'-triphosphate monophosphatase. Belongs to the eukaryotic GTase family.