A term that is often associated with Jay Gatsby in The Great Gatsby is "glamour".
Gatsby is involved, first and foremost, with a dazzling and illusory dream. He fools himself as he fools others because he truly believes in his dream of attaining wealth, glory and true love while living a flawlessly moral life.
Of course, Gatsby's morality is questionable, yet he has wealth and a chance at some kind of love with Daisy. The fact that this love affair simultaneously fulfills the dream and compromises Gatsby's moral position qualifies the situation as one of glamour:
glamour is the impression of attraction or fascination that a particularly luxurious or elegant appearance creates, an impression which is better than the reality.
For Gatsby, the illusion is always prettier and better than the reality.
Interesting question. Indeed the Great Gatsby encompasses many words, it is difficult to pin-point one in particular. However, if I have to choose one I would say the one word that best describes the characters, the plot, etc. would be:
Due to the waste of money, the waste of life, the waste of the time period ("roaring 20s"). I would say that waste describes this story thoroughly, but there are other words that might do the job well, such as: disillusioned, corrupt, materialistic, users