Gatsby deliberately lies and misleads people about his past and history. The money he makes is hinted to be from bootlegging, although it is never verified, and so he is "guilty" only of misleading. However, Gatsby is certainly an "impostor" by definition -- he is not the person he presents himself as, and refuses to come clean about his past. He is not impersonating a specific person, but a type of person; Gatsby, despite having made all his money himself, tries to pass himself off as a person coming from "Old Money" so he will fit in. His purpose is to ingratiate himself with the East Egg residents and so court Daisy, his old love; while she knows his past, she doesn't talk about it, and so helps him keep up his subterfuge. In this sense, Gatsby is not guilty of a specific crime, but of misleading others about his life; he is an impostor, but not for any criminal reasons.