The two most important contrasting settings are the East Egg and West Egg, two superficially identical neighborhoods divided by a small bay.
Twenty miles from the city a pair of enormous eggs, identical in contour and separated only by a courtesy bay, jut out into the most domesticated body of salt water in the Western hemisphere...
(Fitzgerald, The Great Gatsby, mrbye.com)
The West Egg, where Gatsby lives, is considered the "less fashionable" of the two neighborhoods, as the people who live there are mostly rentals and "New Money" residents. People who live in the West Egg tend to have gained their wealth through hard personal work, and do not have long pedigrees or wealthy extended families.
The East Egg, where Daisy and Tom live, is considered the "more fashionable" of the two neighborhoods, as the people who live there are mostly "Old Money," owning their palatial mansions by dint of their inherited wealthy. East Egg residents believe themselves to be of a higher-class than others, and that their family wealth places them above petty concerns.
The contrast between these two neighborhoods is a major source of strife in the novel, as Nick notices and remarks on the uneasy feelings and outright hostility demonstrated by residents on both sides.