Nick, the narrator, starts the novel as an idealist. He strives to the social class of the East Egg, but he is also trying to get ahead on his own merits instead of relying on his own family's general wealth and success. Nick is at first appalled by Gatsby, who stands in stark contrast to the other wealthy families and people living on the Eggs. Gatsby seems contemptuous of the Old Wealth, and Nick is put off by this, since he aspires to that social status. Slowly, as the events of the book unfold, and as Nick becomes comfortable enough with the wealthy lifestyle to see through its outer facade (Jordan Baker being the most important aspect of this realization), Nick starts to admire Gatsby more. Eventually, he comes to admire Gatsby and and feel disdain for the Old Wealth.
When I came back from the East last autumn I felt that I wanted the world to be in uniform and at a sort of moral attention forever; I wanted no more riotous excursions with privileged glimpses into the human heart. Only Gatsby, the man who gives his name to this book, was exempt from my reaction -- Gatsby, who represented everything for which I have an unaffected scorn.
(Fitzgerald, The Great Gatsby, mrbye.com)
This change comes out as Nick sees the essential dishonesty of the Old Wealth compared with Gatsby. While Gatsby lied about his own life, he never lied for cruelty, only to move towards his own goals. The Old Wealth, by contrast, had no concern for anything but their own enjoyment, and abandoned Gatsby as soon as he became a social liability. Gatsby never asked for anyone's help, but everyone around him demanded his attention. Nick comes to admire this individualist ideal more than he admires the Old Wealth society.