Arguably, the quality which makes Gatsby great is that he dedicates his life to making his dreams a reality, which is part of the central idea of the novel. The central idea of the novel is the decline of the American Dream in 1920's New York, as Fitzgerald portrays this time as an era of decayed social and moral values, evidently by the overarching cynicism, greed and empty pursuit of pleasure.
Originally, the American Dream was about discovery, individualism, and pursuit of happiness, but the society of 1920's New York, as depicted in the novel, is about easy money and relaxed social values leading to the corruption of this dream. For example, the party in Tom's apartment, where he has an affair with Myrtle Wilson, expresses the deterioration of morality and culture due corruption and self-indulgence in luxury. However, Gatsby is different, which is expressed by the grand parties where people came "with a simplicity of heart that was its own ticket of admission. Gatsby's conspicuous generosity shows his romantic character (especially as all these parties are only attempts to attract one woman, Daisy) as opposed Tom's tawdry party. Gatsby's pursuit of Daisy, to win her love, is the dream he lives