The previous post is quite lucid. I would only add that I think that Fitzgerald is deliberately indicting the lack of a moral order in the Flapper society of 1920s America. The world that Gatsby moves through is one where, as already pointed out, there are no morals or morality is vague, at best. Part of this represents Fitgerald's indictment of the 1920s, in general. "The Jazz Age," in Fitzgerald's mind, hit all of the wrong notes in trying to establish a moral order, a structure of justice upon which future generations could base their societies. The self- interest and shallow greed of the 1920s was one where the story of Gatsby is both unavoidable and inevitable. The lack of a moral structure is something that Fitzgerald believes lay the groundwork for the economic failures that allows the Great Depression to take place. In Fitzgerald's mind, this economic condition is based on the lack of moral structure or order within the time period, something that is brought out by the characters and setting of the novel.
The world of The Great Gatsby, put simply, is not moral and just at all. Looked at in a more sophisticated and complex way, though, one would say the world of the novel is morally ambiguous.
The character of Gatsby is particularly ambiguous. In his daily actions, he is fair and just. Polite almost to a fault, he is somewhat full of self-doubt. He is polite and thoughtful. His flaw is that he believes an illusion. He dedicates his existence to a return to the past--his relationship with Daisy. But the relationship never really existed as Gatsby experienced it. Daisy never loved him as he loved her. This doesn't make him immoral, however, only deluded and naive.
Yet, he is also an organized crime figure. One can assume that his business in Chicago is not always run with the courtesy and politeness Gatsby uses in his personal life.
Daisy is also ambiguous. She is a woman in a man's world. As she tells Nick, she feels pain for her daughter, since her daughter will face life in the same society she herself lives in. Her only hope of success and advancement will be to be a pretty little fool. In other words, to marry a wealthy man, as Daisy has had to do. It's important to note that when she rejects Gatsby at the end of the novel she rejects him because he asks more than she is willing to give. She won't announce that she has been pining for him all these years and that she never loved Tom. She doesn't reject him because of money--Gatsby has plenty of that.
Yet, she allows Gatsby to be blamed for the accident that kills a human being and slinks away with Tom. Nick the narrator also interprets her character as a manipulator who enjoys messing with people's lives.
Like actuality, these two characters are mixtures of immaturity and maturity, morality and immorality, intelligence and stupidity, good and bad.
As a whole, of course, the world of the novel is not just. Gatsby, who perhaps loves as all people should love, doesn't get the girl and is killed for something he didn't do. The Wilson's don't get what they deserve, either--though they are anything but ideal characters, the consequences they suffer are not just. And Tom and Daisy walk away with their lives fully intact. The novel is not a fairy tale. It's about as just as the real world.