[@Double] Not set in stone:

G's mamma survived the apocalypse
G's mamma and the baby daddy split up or never dated and just had a one night stand
G's mamma became pregnant & gave birth
G's mamma left her with the Mon's that raised G so she could go into battle
G's mamma died

With Kaitou's change in personality after the apocalypse G's mamma could have hidden it from him.

EDIT: With G's name actually being Grogda expect her mother to have something similar.