Write about what you know about has always been good advice for a writer. Fitzgerald constantly drew on his life experience when he wrote whether or not it was a novel or a short story.
He dropped out of university to join the Army and fight in WWI. He met Zelda Sayer and fell madly in love. He was a middle class not yet published writer and she was a true Southern belle.
Fitzgerald drew on this experiences to tell Gatsby's story. Another part of the writer's life can be found in Nick, who just happened to be from Minnesota, as was Fitzgerald.
Like Gatsby, Fitzgerald lived large. He also desired Zelda aka Daisy. Whereas Gatsby was in love with an unattainable illusion, Fitzgerald married Zelda, but that is another story.
Like any good writer, Fitzgerald used his life and his experiences which are then blended into his imagination. As a writer, Fitzgerald gave Gatsby a life of his own within the world he created.