Originally, the title of [Fitzgerald's novel] was “Trimalchio,” based on an ancient satire of a man called Trimalchio who dresses up to be rich.
The idea behind Trimalchio and Gatsby is a similar one. Each of these characters poses as something he is not - rich. Though Gatsby does achieve/acquire wealth, he remains a poseur, playing a role and presenting himself under false pretenses.
Gatsby is not the person he says he is in terms of the personal history he presents in public and to Nick, initially. Gatsby is not even "Gatsby" fundamentally. He is Jay Gatz.
Presenting himself as Jay Gatsby, Gatsby has taken on a part akin to the ancient story of Trimalchio. As a relative of the figure of the roman satire, Gatsby also serves to explore and expose some of the false values of the rich, their immorality, haughtiness and recklessness pride.
Some of the power of Fitzgerald's narrative comes from this allusion to the roman satire. Gatsby is not "just a man" or just a character. In addition to being a developed character in the novel, he is a representative of a type.