I think that one of the major themes of the novel is that the American dream has lost its purity. The author is using the book to argue that the pure American dream of improving yourself has been lost and what they now have is a soul-less pursuit of wealth.
Gatsby represents this process because his story is supposed to be the American dream -- he rises from poverty to become rich and famous. But the way he has done it and his reasons for doing it are not pure at all. He has risen by crime, not by hard (legal) work. And he is not doing it for a good purpose but rather to get the love of a woman who cares only about money.
Fitzgerald saw the '20s as a "lost" time in the US. He thought people were shallow and materialistic. You can see that idea in this book.