Although he is not as wealthy as Tom, Nick comes from the same privileged social set. Daisy is his cousin, and Tom and Nick both attended Yale, where they knew each other. Tom likes and accepts Nick as one of his own. Tom, Daisy and Nick are also part of the same Chicago social milieu, for Tom and Daisy have heard rumors of Nick's supposed engagement to a woman in Chicago.
Tom identifies Nick as a "Nordic" like him, in distinction to Gatsby, who Tom sees as an ethnic and social inferior. Nick, though he comes to admire Gatsby, is also not above poking fun at him on class grounds, laughing at Gatsby for locating San Francisco in the midwest and for claiming he went big game hunting in Europe. Nick may live in a much more modest house than either Gatsby or the Buchanans, but it is a well-equipped cottage, and he has a servant, less unusual then than now, but still a sign of privilege. Nick's father famously tells him not to forget that others may not have had the same advantages as he has.
Money buttresses Nick and allows him to dabble in the bond business, a business he has the freedom to leave once life in New York sours for him. Nick served in World War I, and says he "enjoyed" it "so thoroughly that I came back restless." He doesn't seem to have been traumatized by the experience. His father is willing to support him for a year as he pursues bonds, and Nick is able to make the less practical decision of living on Long Island rather than the city, even affording to do so after the roommate who talked him into the house backs out on him.
All in all, Nick represents the kind of class privilege to which Gatsby can only aspire.