How could modified guanosine be added to the 5' end of the mRNA as a 5' cap ?

GTP is the substrate  NOT guanosine.

Capping process

 The starting point is the unaltered 5' end of an RNA molecule. This features a final nucleotide followed by three phosphate groups attached to the 5' carbon.
 1.One of the terminal phosphate groups is removed (by RNA terminal phosphatase), leaving two terminal phosphates.
 2.GTP is added to the terminal phosphates (by a guanylyl transferase), losing two phosphate groups (from the GTP) in the process. This results in the 5' to 5' triphosphate linkage.
 3.The 7-nitrogen of guanine is methylated (by a methyl transferase).
 4.Other methyltransferases are optionally used to carry out methylation of 5' proximal nucleotides.