What are the main themes in The Great Gatsby?
Love -- love is the motivating force behind many of the characters' actions in the book. It can be argued that infatuation is more Gatsby's motivation and lust is Tom's. I'll stick with Gatsby. There is no doubt that Gatsby is madly in love with Daisy. He goes through all sorts of trouble to woo her away from Tom. On the other hand, maybe Gatsby is just in love with the idea of Daisy.
Deception -- Gatsby is the great deceiver in the book. No one is ever quite sure where he came from or how he made his money. He's obviously trying to conceal his true intentions from Tom, at least early on. Tom is attempting (poorly) to deceive Daisy about his affair. And everybody else is trying to deceive themselves about how much they are dissatisfied with life and the American dream (which could easily be two more themes).
Money and wealth -- There's a lot going on in the book about wealth and money. You have the new wealth vs. old wealth displayed by East Egg vs. West Egg. Add to that the major disparity between any of the wealthy and poor that live in the "Valley of Ashes." There's a great scene where Tom is manipulating George Wilson about fixing his car simply because Tom can. It's significant in itself that the rich call it a "valley of ashes"—not exactly a positive outlook on the poor.