Pitch Gold, how did you become a Guardian?
(Sorry for the late replies) well I sort of put it here in the second part of this introduction fic
In few words, he used to be a sailor but died in a big storm, then he woke up in a beach and not knowing exactly what was happening, until MiM told him he was earmarked for a very important labor (I imagine it sort of happened at acient greece times)
He was selected to be the second guardian