In this passage, Gatsby becomes the archetype of a hero. The "shiftless and unsuccessful farm people" who raised him seemed hardly his parents at all. Like Oedipus who was raised by foster parents, Gatsby creates an identity that is not defined by the parents who raised him. In other words Gatsby leaves his parents and recreates his own larger than life image.
Dan Cody serves as the archetypal mentor who helps the hero find his way. Dan Cody's appearance on Lake Superior resulted in our young hero's transformation from a lowly farm boy named James Gatz to the yachting world traveler named Jay Gatsby. In essence, Cody plays the fairy-godmother and changes our young hero to the young man in a "blue coat," and "white duck trousers and a yachting cap," beginning Gatsby's journey to a role in life more fitting Gatsby's imagination. Perhaps we have a Cinderella story here, because after five years with Dan Cody, the clock strikes midnight, and Jay Gatsby is turned out, never receiving from Cody the money he was supposed to get, having only dreams of attaining the lifestyle to which he had been exposed.
I see the godfather figure in Dan Cody. Cody knows that his lifestyle will ultimately lead him down an unsavory path and maybe a student to his ways could learn from him and ultimately preside over the wealth he might have had and do something good with it. It's as if he saw Gatsby as someone good enough for the family. He saw Gatsby as magnetic. When I say godfather I mean it both in the family sense, and as in the mob mentality. In the Godfather series, there is always a number 2 in the waiting.
I see the mentor in Cody, and the student in Gatsby. Although I am not sure what he learned during those years, I am confident that since Cody took him under his wing and cleaned him up this had great significance in creating a new life for James Gatz, I mean of course Jay Gatsby.
We also see the story archetype of a great tranformation. Because Cody sought him out, Gatz became Gatsby and the birth of new life occur. You could say Gatsby was born again in this moment.
I hope those ideas help.