Gatsby represents a type of romanticism for Nick. Although almost everything about him is false: his name, his college degree, his military record—he’s presented as the only person in the story who has passionate emotions.
By presenting Gatsby as admirable, but doomed, Fitzgerald seems to be commenting as much on the impossibility of that true-hearted romantic spirit surviving in the age he describes. For example, although Gatsby is truly in love, he knows very little about Daisy as a person. It’s only the image of her that he loves.