Fitzgerald relies heavily on symbolism in The Great Gatsby to advance his critique of Old Money society in America. Here are several of his symbols.
1. Automobiles--cars in general represent the carelessness of the wealthy in the book. Ultimately, Daisy kills Myrtle with a car, but even earlier in the novel, almost every circumstance involving automobiles depicts rich people using cars recklessly. In Chapter 1 as Nick provides information about Tom and Daisy through his narration, the reader discovers that Tom had cheated on Daisy, and it caused a scandal because he wrecked a car while driving around with one of his flings. After one of Gatsby's parties, the drunk wealthy leave Gatsby's house and cause several accidents. Finally, Jordan's dishonesty and carelessness is linked to cars. She borrows a friend's car and lies about leaving the top down which ruins it, and she nearly hits someone while she's driving with Nick.
2. The green light--At the beginning of the novel when Nick sees Gatsby reaching out toward something (Daisy's green dock light), the light symbolizes hope. Gatsby's dream is just out reach, but he feels that he will attain it soon. At the novel's end, Nick mentions the green light, and it is simply the corrupted American Dream. It is still there, but once someone achieves it, it has lost its luster. Additionally, it is still across the water, out of reach.
3. White--Daisy most often wears white, as do some of the other wealthy. The color represents Old Money folks--they can afford to wear such a light color because they do not have to worry about working or soiling their clothing. Moreover, Daisy's connection with the color is that for Gatsby she represents all that is pure. He wants to go back to when he met her as a young socialite wearing white and relive the past. He does not realize that she and, thus, his dream have been corrupted.
There are numerous other symbols in the novel such as the Valley of Ashes, the billboard, Owl Eyes, etc. In answer to your second question about setting, its role is all important to the novel. Fitzgerald was nicknamed The Chronicler of the Jazz Age because he depicted his era so thoroughly. Through Gatsby he portrays struggle between those who inherited their wealth and those who "worked" for it. He also discusses the effects of the Roaring 20s with its materialism, bootlegging, and lavish parties upon Americans.
Setting plays an important role in developing some of Fitzgerald's themes. East Egg, West Egg, and the Valley of Ashes all assume symbolic significance. East Egg, the home of the Buchanans, symbolizes the wealthy upper class that lives on inherited family money, handed down through several generations. This old money has created a class of people who enjoy the highest social status and who associate mostly with each other. They have gone to the best schools together, done business together, and follow the same rules of behvior and social convention. Fitzgerald portrays them as being arrogant, snobbish, and selfish, living life with an inbred sense of entitlement.
West Egg, home to Gatsby, is populated by those who have earned their great wealth. It symbolizes the new rich who built fortunes at the turn of the century and during the boom of the early 1920s. This new money lacks social acceptance, and those who possess it are considered by the elite in American society to be gauche and crass. They lack "breeding."
The Valley of Ashes, where George and Myrtle live, symbolizes the grinding poverty of the underclass in American society and the industrialization that built American capitalism at the expense of some. This awful, dirty place with its pervasive, enveloping smoke and ash-filled air contrasts starkly with the lush, green beauty of both East Egg and West Egg.
Also, the East and the West take on symbolic meaning. The East symbolizes the corruption of the original American Dream; the West is symbolic of America's history and founding ideals, all that once made it great. Nick, as a midwesterner, serves as Fitzgerald's voice of midwestern morality. When Nick rejects the East and the corruption it stands for, choosing to abandon his career and go back home, one of Fitzgerald's major themes is made clear.
- the green light- gatsby reaching for daisy; hopes/dreams of money to buy daisy's love
- green- hope (pg 189, "Gatsby believed in the green light...")
- gold- wealth (pg 11, "glowing now with reflected gold")
- white- pure (pg 24, "white girlhood")
- yellow- corruption (pg 147, "big yellow car")
- pink- embarrassment (pg 162, "gorgeous pink rag")
- silver- wealth (pg 151, "silver scales")
- blue- sadness (pg 43, "blue gardens")
- red- love, shame, rage (pg 22, "crimson room")
- east egg/west egg- east egg: old money, upperclass; west egg: new money; egg- white outside (pure), yellow inside (corrupt). (pg 9, "west egg less fashionable...")
- cars- abuse of power/money (pg 63, "you're a rotten driver"
- car accidents- general air of carelessness and moral abandon (pg 60, "at least a dozen men...")
- weather- represents mood and events; heat- tension rising (pg 120, "the next day was broiling...")
- molars- shows criminal side of wolfsheim (pg 77, "i see you've been looking at my cuff buttons.."
- falling clocks- Gatsby is trying to get his time back with Daisy (pg 91, "luckily the clock took this moment...", pg 116, "you can't repeat the past...")
- falling phonebook- foreshadows that everything is going to fall apart (pg 133, "the telephone book stopped...")
- valley of ashes- represents the decay of society, the lowest of the low (pg 27, "this is a valley of ashes...")
- t.j. eckleburg- the eyes represent God watching over the valley (pg 27 "but above the gray land...", pg 167, "i told her she might fool me...")