In Chapter 4, he spends more time with Nick, telling him about his service in WWI as well as a made-up story about his past as the only surviving member of a wealthy family.
Later, he has Jordan explain Gatsby and Daisy’s background in a bid to get Nick to help the pair reunite.
It understood you just so far as you wanted to be understood, believed in you as you would like to believe in yourself and assured you that it had precisely the impression of you that, at your best, you hoped to convey.
Precisely at that point it vanished--and I was looking at an elegant young rough-neck, a year or two over thirty, whose elaborate formality of speech just missed being absurd.
Through Jordan and Nick, Gatsby is thus able to meet with Daisy again and begins an affair with her in Chapter 5.
Throughout all of this Gatsby continues to do business with Meyer Wolfsheim and run his own bootlegging “business," mainly based on the mysterious phone calls he's always taking.
After the war ended, he briefly attended Oxford University through a program for officers, but left after five months.
By the time Gatsby returned to America, he learned that Daisy had married and became determined to win her back.
However, although Cody intended to leave his fortune to Gatsby, it ended up being taken by Cody’s ex-wife Ella Kaye, leaving Jay with the knowledge and manners of the upper class, but no money to back them up.
Gatsby ended up enlisting in the military during World War I.