Flashback is used at various points in The Great Gatsby to provide background information and insight into the characters.
Nick learns the history of Daisy's acquaintance with Gatsby and of her marriage to Tom through a flashback narrated by Jordan Baker. Nick introduces the presentation
One October day in nineteen-seventeen - (said Jordan Baker that afternoon, sitting up very straight on a straight chair in the tea-garden at the Plaza Hotel) - I was walking...
Another use of the technique fills in the factual life history of "James Gatz of North Dakota." Nick gains this information partially as the result of investigations by "an ambitious young reporter from New York" and partially as told by Gatsby himself, "very much later."
Gatsby indulges in the use of flashbacks to keep alive his fantasy that someday he and Daisy will be able to shed other connections and complications and build a future together. After the dance at his house, Gatsby talks with Nick. He expresses his regret that Daisy didn't enjoy the party, then begins remembering earlier and better times in their relationship. When Nick urges him to stop, saying, "You can't repeat the past!", Gatsby's immediate reply is, "Can't repeat the past?...Why of course you can!"