What are some of the most important symbols in The Great Gatsby, and what role does setting play in the novel?

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.