Gatsby is not particularly truthful, or at the very least, he is guilty of lying by omission. He can't tell the true and complete story because he is creating a persona based on what he wants to be, which is the scion of a family of old money, the person he believes will win Daisy Buchanan over. In reality, he was born into a poor North Dakota family, and dropped out of college in Minnesota where he was supporting himself as a janitor, a situation of which he was ashamed. He spends a decade with a copper tycoon learning the tricks of the wealthy trade, serves in World War 1, and then, he tells Nick, he attended Oxford University in England, following in the footsteps of his illustrious family--a major untruth. Back in America, Gatsby makes friends with the right gangsters and subsequently amasses a fortune in bootlegging profits, which he uses to purchase a mansion across the water from the Buchanans, where he throws wild parties every Saturday night in the vain hope that Daisy will come some weekend.
(To lhc): Why does Fitzgerald tell the story of Gatsby now and not earlier or later?