Dan Cody is essentially the man who made James Gatz become Jay Gatsby. After dropping out of college, Gatsby returned to Lake Superior, where he had grown up. While lounging on the beach one day, he met Dan Cody and his yacht.
Cody was fifty years old then, a product of the Nevada silver fields, of the Yukon, of every rush for metal since seventy-five. The transactions in Montana copper that made him many times a millionaire found him physically robust but on the verge of soft-mindedness, and, suspecting this, an infinite number of women tried to separate him from his money...He had been coasting along all too hospitable shores for five years when he turned up as James Gatz’s destiny at Little Girls Point.
Nick goes on to say that Gatsby won Cody over with his smile, & Cody decided to take him along when the yacht left harbor. He was employed by Cody for 5 years as "steward, mate, skipper, secretary, and even jailor, for Dan Cody sober knew what lavish doings Dan Cody drunk might soon be about". It's clear that Cody has a problem with liquor, which is why Gatsby himself never drank at his parties. Nick further describes Cody as
a gray, florid man with a hard, empty face—the pioneer debauchee, who during one phase of American life brought back to the Eastern seaboard the savage violence of the frontier brothel and saloon.
This description of Cody hints of criminal activity, so Cody may have inspired Gatsby's bootleg business as well. This may have also been because although Cody left Gatsby $25,000 when he died, Gatsby never got any of it. Thus, with no resources and only a new name, Gatsby turned to crime to make his fortune for Daisy.