GB didn't exist before Mario's Madness.
It was once thought that GB stood for Game Boy, but it's since been confirmed that it is just GB, like how MX is just MX.
GB was made because Marco Antonio wanted to make an EXE based on his favorite Mario game. He also comments that "GB was created while i was s****ing lol".
GB and Horror Mario used to be the same person, but that has since changed, and he is his own character.
GB can travel to other Mario games and corrupt them.
GB can even go to newer M