It’s the illusion of Gatsby that is great; what his life is like to a stranger looking from afar. He seems to have it all, giant house, handsome, big parties, etc. and he wants people to believe he is as great as his things. But given the end, we know he isn’t really great and he might have “it all” but without Daisy he has nothing (nothing that matters at least) and he realizes he’s isn’t great.
