Explain how Fitzgerald uses setting to emphasize the differences between the social classes.
Instead of living "across the tracks," Nick Carraway lives across the lake from the Gatsby mansion, which he can see lit up at night during Gatsby's famous VIP parties. The water stretching out between them and the difficulty of access suggest the social gap between them as well. Although Gatsby befriends Carraway, Nick nevertheless remains a spectator of the New England upper crust and never really belongs.
The colour green as a symbol of reclusion, exclusion, wealth, desire (envy and lust), and disenchantment is also a predominant leit motif throughout the novel. See the enote reference below in this regard.
One's origin (an aspect of setting) also plays an important role in this story. From enotes 'Critique of American Upper Class Values':
'For the “old” (inherited) money crowd, family lineage is often the first, and perhaps most important, indicator of class rank. This theme runs through the entire novel. Tom’s old Chicago family is “enormously wealthy.” In fact, “his position” was what attracted Daisy to him....
Gatsby, one the other hand, is of unknown background. Rumors circulate that he is related to everyone from the Kaiser to Satan. Eventually we learn that Gatsby comes from a humble, midwestern family. He grew up poor.
The rivalry between the established families and the nouveau riche is another aspect when considering setting as it establishes the social backdrop of the story.
Setting as a time period also plays an important role in this novel. Enotes essay 'Romance and Cynicism in The Great Gatsby' explains:
Fitzgerald presents romanticism and cynicism in the Jazz Age as two sides of the same coin, and as two forces that can never be reconciled. The Buchanons and Jordan never seemed to have had dreams beyond attaining success and status; Gatsby’s dreams prove to be his undoing; and Nick’s dream of success in New York dissolves into a cynical mess he can only escape by leaving.
Through these lives, Fitzgerald seems to be telling us that romantic ideals are impossible in early 20th-Century America, that they are a relic of a bygone era. He also appears to mourn that era, throwing all his characters into a world where no one can trust anyone else and no good deed goes unpunished.