From the pessimistic angle, "The Great Gatsby" is a satire of The American Dream. Daisy, always dressed in white with a white roadster is blank and transparent--ethereal like the age-- becoming whatever one perceives her as. After her husband discovers her telling Gatsby "I love her," he looks at his wife "as someone he knew a long time ago." The remark that Daisy's voice "is full of money" comes from Gatsby, for that is his perception of his "golden girl," Daisy. And, because of this perception, Gatsby believes that he must win Daisy with his wealth. This wealth, then, is equated with the American Dream, a dream that turns false with Gatsby becomeing a truly tragic figure.
The overwhelming materialism of the era is clearly satirized by Fitzgerald as he describes the decadence of the people--the "twins in yellow dresses"--attending the parties of Gatsby. His car is, indeed, symbolic of the love of materialism and the conferring of power to the acquisition of such material things with its fenders like wings and
a labyrinth of windshields that mirrored a dozen suns....bright with nickel.
Such symbols remind the reader of the excesses of the 1920s, the shallowness of the people who sought only material possessions and money as their goals and the disillusionment that follows.
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.