The Great Gatsby is a very interesting story set in the 1920s that exhibits things like love, fame, and fortune. The story reveals a tale of love that can never grow between Gatsby and Daisy but even bigger than that is the theme of the demise of the American dream.
A theme is defined as a main idea or an underlying meaning of a literary work (You can read more about themes here, on literarydevices.net). Deeper than the love story of The Great Gatsby though, is the metaphorical death of the American dream in the area of Long Island, New York. In the 1920s, America was recovering from World War I and Fitzgerald paints a picture of a city celebrating wildly, with complete abandon and lack of restraint. In the novel, new money (meaning, people who earned their wealth alone) was easy to come by through industrialism and the stock market, while old money (families that had been wealthy for generations) did not welcome or like the newcomers. This idea is evidenced by the clear separation of East Egg and West Egg.
In Chapter 9, Nick discusses the foundations on which America was built on: individualism, ingenuity, and the pursuit of one’s own happiness. However in Fitzgerald’s 1920s, the decline of social morality and the ability to grow new wealth have corrupted the American foundations.
In the end, Gatsby learns he can never have Daisy and even though he is wealthy, he sees nothing left to do but die without her. Like Gatsby’s love, the values of America disintegrate to the point of disrepair, according to Fitzgerald. Gatsby’s love for Daisy will never come to fruition, just as the American dream cannot come back from the material excess and financial abandon with which New Yorkers operated in the 1920s.
I hope that helps! Please comment back if you have further questions.