Gatsby's mansion is located on West Egg, a fictional town modeled on Great Neck. Author F. Scott Fitzgerald rented a house in Great Neck for his wife and daughter and observed it as a working-class area dotted with mansions of the newly-wealthy who, during the 1920s, often threw lavish parties. East Egg, the home of Tom and Daisy Buchanan, is based on Sands Point, a town along Long Island's "Gold Coast" where some of the most opulent estates of the American elite could be found. East Egg is more exclusive than West Egg—it takes more than money to have a house there. One must have the right social connections and background to be accepted by East Egg society.
The bay separating East Egg and West Egg is analogous to the social gulf that separates Gatsby from the Buchanans. While the bay is narrow, allowing him to see the green light on the Buchanan dock, it is essentially unbridgeable. Gatsby has the money, the home, the cars, the clothes, and even the speech patterns and affectations of a man in Tom Buchanan's class—but he was not born into wealth.
In many ways, this makes Gatsby the better man. Nick Carraway expresses as much the last time he sees Gatsby. Fitzgerald's tragic hero, after all, is a man of ideals and vision who has overcome so much and definitely achieved more on merit than Tom Buchanan. The fact that Tom Buchanan was born rich, however, gives him an in-borne sense of superiority. He looks down his nose at Gatsby's parties as "circuses," which he could throw if wanted to—after all, he gave Daisy a necklace worth over $350,000 (in 1920s money) as an engagement present.
Tom's smug sense of superiority is the weapon he uses to defeat Gatsby. When Tom points out at the Plaza Hotel that Gatsby is a bootlegger, Daisy withdraws her love and abandons him. She does this not because of any moral outrage but because Gatsby is no longer able to maintain the illusion of himself as Old Money anymore than he can uproot his mansion and move it to East Egg. Tom has pointed out not that Gatsby is a criminal but that he's nouveau riche—he's a West Egger, a trespasser trying to swim across the bay.