This is an age old theme in literature. Illusion / Reality is known as a “dichotomy,” which means two terms that are opposite to each other, but which create an interpretive tension. Literature is filled with dichotomies, and authors use them to create meaning: light / dark; good / evil; war/ peace; male / female; life / death. There are hundreds of them. A very effective way to understand and interpret literature is to locate the different dichotomies, and try to understand why the author is using them.
So Fitzgerald uses the dichotomy of Illusion / Reality throughout the entire novel. In context of the issue of money, Fitzgerald shows a world in which wealth creates veils darker human nature. The beautiful mansions hide ugliness inside. The same holds true for people. Although Jordan Baker is beautiful and outgoing, she reveals herself bit-by-bit to be uncaring and ruthless. Centrally, Daisy Buchanan is beautiful, vivacious, friendly and elegant. She comes across to the reader as being a positive and attractive person.
But as the novel progresses, Daisy manifests her carelessness, selfishness and apathy. Finally, at the end of the novel, she not only lets Gatsby take the fall for murder, but flees the east coast with Tom without a return address, so to speak. Jay Gatsby encapsulates the dichotomy of illusion / reality the most. His whole “aristocratic” pose is a front for his criminal operations. His slight British accent is feigned. In the first few chapters of the novel, Gatsby remains a mysterious figure.
We only hear snippets of information about him from various people that may or may not be trues, such as: he is friends with the Kaiser; he killed a man once when he was young; he went to Oxford; he inherited piles of money from German descendants. In fact, we never know the complete truth about Gatsby, except for the story of his life he tells both Jordan and, later, Nick, and Myers Wolfsheim’s story of how Gatsby came to the underworld (which may, ironically, be the only truthful story about him). Notice how in many of the “party” scenes, at one moment the party looks glamorous, and the next it looks cruel and tawdry.