Evidence abounds in support of the argument that many of the characters from the novel are liars or engaged in deceit.
The prime example is Gatsby. Jay Gatsby has changed his name and moved to the east coast where he lives as a bootlegger (e.g., a criminal). His back-story is partly true and partly made up, as he relates it to Nick initially.
Born to wealthy parents in the Middle West—all deceased now—he had been educated at Oxford and, after inheriting his family’s wealth, had lived “like a young rajah” in Paris, Venice, and Rome, collecting rubies, dabbling in hunting...
It's true that Gatsby was born in the "Middle West", but the rest is a lie. Though Gatsby adheres to a dream and maintains a semblance of innocence, he is none-the-less outwardly a fraud.
Gatsby is perhaps the most obviously fraudulent character, but he is not alone in this category.
The wealthy class is morally corrupt in The Great Gatsby...
Many of the other characters also engage in deceit and lie. Tom cheats on Daisy. Daisy prepares to run off with Gatsby. Myrtle cheats on George Wilson. Jordan is caught cheating at her sport.
Justified by a culture that seems to promote an unconditional pursuit of "material happiness", these characters define the moral code of their class.