Fitzgerald's novel is remarkable for a number of reasons, one of which being an achievement of layered characterization. Characters in the novel are developed in multiple ways, through direct narrative description (often through poetic language), through symbolism, through dialogue, and through hearsay, gossip and reputation.
The final method of characterization on this list is significant in its signification of the world that Fitzgerald has created in the text. Characters know of each other and this vague, reputational knowledge is used to influence the reader's view of characters, especially Gatsby.
This approach to characterization allows for ambiguity in characters as well as depth. As we see in Gatsby, there are characters described one way through gossip who later we find to be different from what the rumors suggest - but not completely different. There is a grain of truth to the gossip, which complicates the direct descriptions offered by Nick, the narrator.
Though Nick comes to see Gatsby in a way that is distinct from the way most people do, Nick's view does not become the "real view" of Gatsby, as demonstrated by his empty funeral party.
The layered development of characters also includes uses of symbols and tags that are associated with characters. Jordan is cool, Daisy has a voice like money, and Tom is a brute built like a house. These ideas become part of the character description and part of what the characters represent as well.