We might rephrase this prompt as a question - "How is morality a complex issue in the novel and what character best exemplifies this complexity?"
Another interpretation could be phrased like this: How does the notion of moral ambiguity in the novel relate to this line uttered by Nick?
"Reserving judgment," he says, "is a matter of infinite hope."
How does this notion of hope (for people to prove their worth/moral value) play into the novel in relation to Gatsby, Daisy, Nick, etc.?
There are a number of potentially important implications to the answer to these questions. If Fitzgerald's novel refuses to allow us to see the story as a "morality tale" with a lesson at its center (despite the fact that moral judgement is so critical to the text), we can see the novel as questioning conventional views of morality. In other words, despite the fact that Gatsby is judged by Nick repeatedly, he is not consistently seen as "good" or "bad", moral or immoral. He is too complex to be simplified like this.
So, what is the meaning of his character? The answer will necessarily be related to the themes of the novel relating to ambition, posturing/superficiality, the American Dream, etc.
Some other ideas to consider with this prompt:
- If Gatbsy's morality is ambiguous, does that make the generic American Dream morally ambiguous?
- In what ways is this a book about the impossibility of judging those who we don't understand? In what ways is it a cautionary tale about moral judgement in general?
- Is one of the novel's points a comment on the slippery moral complexity of all people, considering that even Nick is not as pure hearted as he claims and his friends are all liars, cheaters, and adulterers?
Hopefully some of these ideas help you to see the prompt as a somewhat open and rich essay topic (that gives you plenty of material to work with from the novel).