Jay Gatsby was not always a wealthy and well-connected man. Instead, he was originally named James "Jimmy" Gatz and was a poor farm boy from North Dakota. He was admitted into college but had to work as a janitor to afford it, and finding that work too humiliating, he traveled to the shore of Lake Superior, and supported himself by fishing and odd jobs.
Dan Cody was a self-made man who made a fortune in mining and had moored his yacht near where Gatsby was working. Gatsby noticed a storm brewing and rowed out to warn Cody of the storm. In gratitude, Cody offered Gatsby a job, and Gatsby became a general assistant to Cody. Cody was a mentor to the young Gatsby and in many ways a role model, except in one way. Cody was a very heavy drinker, and Gatsby saw that as a major weakness, and tended to avoid alcohol. When Cody died, he left Gatsby $25,000 (the equivalent of $330,000 in 2016). Gatsby's investment of this money was the source of his wealth.
This is a good question, because Dan Cody is a man that set Gatsby's life on a certain trajectory.
According to the novel, Gatsby went to college, but because he grew up on a poor farm, he had to work his way through school. He worked as a janitor. He could not put up with the humiliation of this, so he left. Instead, he worked on Lake Superior fishing and digging clams.
One day he met Dan Cody, a wealthy man on a boat. Gatsby warned Cody of an impending storm. This was when the two became friends. Also Gatsby became Cody's personal assistant. When Cody died, he left Gatsby a large sum of money, $25,000, but Cody's mistress did not allow Gatsby to have it.
Gatsby at this point set himself to make money, and money he did make. Part of his ability to make money was on account of the time he spend with Dan Cody. From this perspective, we can say that Gatsby would have not been the Great Gatsby without the "Great" Cody.
This question has already been answered. Here is a comprehensive link for you: http://www.enotes.com/great-gatsby/search?q=dan+cody
Dan Cody is Gatsby's "Role Model" when Gatsby first got out of the war he was poor and Dan Cody taught him how to make a living and not be homeless and stuff like that anymore