who is dan cody and what is his significance in gatsby's life?

Dan Cody was an enormously wealthy old man, a millionaire many times over, who had earned his fortune mining silver. Gatsby met him when Gatsby was seventeen--a runaway with no money and no prospects. When he saw Dan Cody's yacht drop anchor at a dangerous point in Lake Superior, Gatsby saw an opportunity to leave his old life behind and took it:

It was James Gatz who had been loafing along the beach that afternoon in a torn green jersey and a pair of canvas pants, but it was already Jay Gatsby who borrowed a row-boat, pulled out to the Tuolomee and informed Cody that a wind might catch him and break him up in half an hour.

Cody found Gatsby to be smart and ambitious and hired him. Cody fitted him out with a nautical wardrobe, and the Tuolomee set sail for the West Indies and the Barbary Coast with Gatsby aboard. For five years, Gatsby worked as Cody's personal assistant, of sorts, as a "steward, mate, skipper, secretary and even jailor [sic]." (Gatsby "jailed" Cody when he was drunk and needed to be restrained.) Together they sailed around the continent three times.

During the time he travelled with Cody, Gatsby experienced a glamorous life far removed from his North Dakota upbringing. He attended parties with the wealthy where women were known to "rub champagne into his hair." He acquired a certain amount of polish and sophistication. Most of all, during his time with Cody, Jay Gatsby left Jimmy Gatz behind: "[T]he vague contour of Jay Gatsby had filled out to the substantiality of a man."