This novel is "about" a number of things. Readers and critics often comment on the book's depiction of the "American Dream", the complex ambitions of a naive criminal dreamer (Jay Gatsby), and the immorality of the wealthy.
In its use of symbolism and motif, Fitzgerald's novel takes on several layers of meaning that make the book about more than what is just on the surface, yet one of its themes is certainly related to the superficial, surface-oriented values of the upper classes. Many of the wealthy characters act immorally in one way or another.
Gatsby is a criminal (bootlegger). Tom commits adultery (repeatedly with different women) and he is violent, hitting Myrtle at one point. Daisy cheats on Tom with Gatsby and denies responsibility for killing Myrtle in Gatsby's car. Jordan cheats at her sport, causing a small scandal. Wealth and the privilege that comes with it are shown as morally corrupting in the novel.
This is one way to see the novel's commentary on the American Dream. While for many this dream is one of self-improvement, the characters who seem to be examples of achievement - or at least "success" - are not "improved" people. They are bad people. They are worth something only materially.
Despite the bad behavior and apparent moral corruption of material success, the characters continue to be drawn to the allure of such a dream. Daisy comes to represent this romance of wealth, to Gatsby and to Nick as well.
“Her voice is full of money,” [Gatsby] said suddenly.
That was it.… That was the inexhaustible charm that rose and fell in it, the jingle of it, the cymbal’s song of it.…
What can we say about this group of people and about Gatsby in particular when we see them choosing to believe in what becomes clearly a fantasy (of wealth, exceptionalism, the romance of wealth, etc.)? One thing that people comment on is the fact that Gatsby's willingness to believe in the dream is suggestive of an innocence on his part.
Despite the fact that he has made his riches through illegal activity, he continues to believe in concepts like love and honor, which lead him in part of pursue a marriage with Daisy.
Gatsby commits himself to “the following of a grail” in his pursuit of her and what she represents. This obsession is characteristic of a dreamer like Gatsby, who loses a sense of reality but rather believes in “a promise that the rock of the world was founded securely on a fairy's wing.” (eNotes)
We follow Gatsby through the end of his life as he pursues his ambition and his romance, not only with Daisy, but with the concept of success. To reach his goal he poses as something he is not. He carries on an elaborate ruse, complete with swank parties and big cars. He enlists the help of his poorer neighbor, Nick, and in doing so ultimately finds defeat. Yet, through it all, he never stops believing that he will marry Daisy and fulfill his great potential.
It is a book about self-deception then and about the power of personal convictions. It is about moral corruption and about dreams.