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I lived at West Egg, the—well, the less fashionable of the two, though this is a most superficial tag to express the bizarre and not a little sinister contrast between them. [...] Across the courtesy bay the white palaces of fashionable East Egg glittered along the water, and the history of the summer really begins on the evening I drove over there to have dinner with the Tom Buchanans. Daisy was my second cousin once removed, and I'd known Tom in college. And just after the war I spent two days with them in Chicago. (1.14-15)
Nick sees two kinds of America: the hard-working Chicago, part of a "Middle-West" culture; and the "white," fashionable East Egg. Nick may be able to make it in the Middle-West, but he's not cut out for East Coast life.
I decided to call to him. Miss Baker had mentioned him at dinner, and that would do for an introduction. But I didn't call to him, for he gave a sudden intimation that he was content to be alone—he stretched out his arms toward the dark water in a curious way, and, far as I was from him, I could have sworn he was trembling. Involuntarily I glanced seaward—and distinguished nothing except a single green light, minute and far away, that might have been the end of a dock. When I looked once more for Gatsby he had vanished, and I was alone again in the unquiet darkness. (1.152)
We never see Gatsby's version of America directly; we just get glimpses of it through Nick's narrative and through ephemera like his childhood notebook. But, judging from the way he's staring across the water, Gatsby has a pretty spectacular vision.
He was balancing himself on the dashboard of his car with that resourcefulness of movement that is so peculiarly American—that comes, I suppose, with the absence of lifting work or rigid sitting in youth and, even more, with the formless grace of our nervous, sporadic games. This quality was continually breaking through his punctilious manner in the shape of restlessness.
He was never quite still; there was always a tapping foot somewhere or the impatient opening and closing of a hand. (4.12-13)
Gatsby is one version of America—the resourceful, athletic, restless young nation striving to make itself better. The problem is, America as Nick sees it isn't like that anymore. It's beaten down, like George Wilson; or it's rich and careless, like Tom. Does that make Nick the happy (or unhappy) medium?