During their ride into New York, Gatsby spins some very fantastic tales about his personal history for Nick. All of his stories are designed to make him seem a worthier and more respectable man, in spite of the rumors Nick surely has heard. To emphasize his sincerity, Gatsby refers to what he tells Nick as being "God's truth."
According to Gatsby, he was born into a wealthy Midwestern family, all of whom were (conveniently, for his purposes) dead. After growing up in the United States, he went to school in England, at Oxford, in accordance with the long-standing family tradition. When questioned by Nick, Gatsby adds, surprisingly, that his Midwestern home was San Francisco. After all his family died, he inherited the family fortune, moved to Europe, and lived "like a young rajah."
His story continued that his life in Europe ended when World War I began, he served as a first lieutenant in the Argonne Forest, and performed incredible feats of military heroism. For his service, he was received a military promotion to major and was decorated by all of the United States' allies, including "little Montenegro down on the Adriatic Sea!"
Nick finds the story to be incredulous, but is then shocked and confused when Gatsby shows him two souvenirs from his past: an inscribed, authentic-looking medal from Montenegro and a picture of Gatsby taken at Oxford with a group of young men in blazers: "There was Gatsby, looking a little, not much, younger--with a cricket bat in his hand." Untangling the truth of Gatsby's past requires the rest of the novel. Gatsby tells Nick this fantastic history to establish his personal credentials and gain Nick's approval because Gatsby wants Nick to arrange for him to meet Daisy.