Loss of God
The eyes of Dr. T.J. Eckleberg look down upon the adulterous, lying, wealth-obsessed characters in The Great Gatsby like the eyes of God. Fitzgerald is implying that wealth has become the new God of the material world. The characters have only one faith: in money. The do not trust each other, they do not trust in God, they do not even trust themselves. Their lives are lacking in direction - except for Gatsby. He is striving for the new Holy Grail - wealth and acceptance by the aristocrats he believes to be his friends. It is possible to see Gatsby as a figure of messianic qualities, drawing people to him from all around. When the messiah is discredited, he suffers an ignoble and painful death.
The American Dream
Gatsby is representative of the American Dream - the notion that the United States is a meritocratic institution where anyone can become a success through hard work and intelligence. The novel is remarkable in that all of the principal characters are Midwesterners who have come East to find or consolidate their fortunes. Gatsby's dream is based on lies and, much like Fitzgerald, having achieved his aspirations, he is left a drained man. Gatsby believes that Daisy will only love him if he has the material wealth to support her in the fashion to which she is accustomed. This mirrors Zelda's refusal to marry Scott until he is wealthy enough for them to live the high life. The dream-state of owning gold and silver, yachts, and motorcars is revealed to be a hollow world. Supremely emblematic of this is Gatsby's library which is vast but where none of the books' pages have been cut - they are all for show, like so much in his life. It is notable that it is a gold car that kills Myrtle, and as such it is a combination of false gold and fast cars that brings about Gatsby's demise.
Gatsby's Greatness
Gatsby possesses qualities which other characters in the novel cannot aspire to. He is full of hope, has an unjaded sense of the romantic, and maintains a form of nobility throughout his doomed life as a pseudo- aristocrat. He also has a wonderful sense of the past, remembering with fondness his days of being a no one. In many ways, of course, the title is an ironic one. Gatsby's real name is Jay Gatz, and his ignominious death marks him out as far from 'great'. But the reader, like Nick, sympathizes with Gatsby's ambition, and the fact that he has achieved so much of what he set out to do. He even manages to possess Daisy for a few heady days, before his lack of substance is revealed and he is rejected by the society which fostered him.