As the novel develops, Nick observes Gatsby and discovers bits and pieces of the truth about his past, until all of it is eventually revealed. Along the way, Nick is drawn into a personal relationship with Gatsby, who takes Nick into his confidence and shares his deepest feelings. Nick's perceptions of Gatsby change as he becomes deeply involved in Gatsby's life. When he first met Gatsby, Nick viewed him as a larger-than-life presence in West Egg, a young man about his own age who seemingly had "drifted" from nowhere into fabulous wealth. By the time Gatsby died, Nick had come to perceive him as a romantic who had dedicated himself to an impossible dream.
In Nick's estimation, what made Gatsby "great" was not his enormous wealth or extravagant lifestyle, but the purity of his heart in pursuing his "colossal dream" of loving Daisy. Gatsby was great in that his romantic dream itself was great, to take control of life, repeat the past, and write a new fate for himself. Since the novel is structured as a flashback, Nick introduces himself in the beginning and makes his ultimate perception of Gatsby very clear:
. . . there was something gorgeous about him, some heightened sensitivity to the promises of life . . . it was an extraordinary gift for hope, a romantic readiness such as I have never found in any other person and which it is not likely I shall ever find again.
Jay Gatsby, formerly Jimmy Gatz of North Dakota, had come "a long way" and at the time of his death, "his dream must have seemed so close that he could hardly fail to grasp it." For Nick, it was Gatsby's formulation of that dream and his dedication to it that made him great.