What is the function of the symbolism in The Great Gatsby?
Symbolism allows the novel to be read with multiple perspectives.
First, it can be read by a feminist angle: women are characterized as symbols, objects of men, temptresses, and materialistic gold diggers. They are "hopeless little fools" whose voices are "full of money" but they don't say anything. They are careless (Jordan), careless drivers (Daisy), careless lovers (Myrtle).
Second, the novel can be read by Marxist critics. Look how social classes are depicted in the novel: there's the elite East Eggers (the bourgeoisie) from the established East Coast, the West Eggers from the west, and the Valley of Ashes (the proletariat) caught in the middle. Nick, the narrator, is from the Midwest; though he's from the proletariat he likes the fact that Gatsby has joined the bourgeoisie despite his criminal means to the end. There's a definite Midwestern bias in his narration, and all the symbols are colored accordingly.
Third, the novel can be read by Jungian myth critics who see the same symbolic stories in Gatsby that have been told throughout history. Daisy is a siren to Gatsby's Odysseus. Nick is a Nicodemus to Gatsby's Jesus. Gatsby is a Bryonic hero whose desires are so focused that he doesn't see his death coming.
Of course there's the color, clothing, and geographical imagery. Fitzgerald uses heavy symbolism and metaphor to allow the novel to transcend time, to be a quintessentially optimistic and pessimistic American novel. In Gatsby, America is both full of dreams and false promises, depending on where you live, who you are married to, and what kind of car you drive.