In The Great Gatsby, honesty is a huge theme, as no one is quite what they seem. Be it Tom who plays the devoted husband, yet leads Nick to his apartment in the city where he has an open affair with a girl from the Valley of Ashes, or Daisy who appears weak and defenseless, yet later on runs over her husband's mistress in Gatsby's car, characters are very multidimensional.
Gatsby refuses to give straight answers, and it is not until after he all but loses hope for achieving his goal of winning back Daisy that he comes close to being honest with Nick, the narrator. In fact, Nick doesn't get the whole picture of Gatsby's life until after he is dead, and he has an opportunity to speak with his father, Mr. Gatz, at a very poorly attended funeral.
No one is what they seem. Jordan cheats at Golf, while Tom, Daisy, and Myrtle all have extramarital affairs. When Nick tells Gatsby that he's worth "the whole damn lot of the put together", he is making a valid judgement call in that no one else ever comes clean and exhibits true honesty except for Gatsby in the end. This adds a lot of meaning to the quote early on in the novel where Nick says, "No, Gatsby turned out alright in the end... it was what preyed on him" that finally got to him.
Honesty is the most prevalent theme in The Great Gatsby because it dominates every aspect of the novel. Specifically, it is the lack of honesty which really comes to the fore in this story. Jordan Baker, for example, is famed for cheating in golf, and Tom and Myrtle are both lying to their partners while they enjoy an affair. Even Daisy Buchanan's persona is one based on lies: she confesses to Nick in Chapter One that everything is "terrible" but would never openly admit this to other people around her.
Gatsby, too, lives a life dominated by lies. He built his fortune using lies and deception, for instance, and hosts his parties for Daisy's benefit, not to impress or entertain others. As such, honesty and lies drive the plot of the The Great Gatsby.
Honesty is also responsible for creating the major turning points in the novel. When Daisy openly (and honestly) chooses her husband over Gatsby, for instance, Gatsby's dream is officially over. Moreover, when Wilson discovers his wife's affair, this leads him to murder Gatsby and then take his own life. It does not matter that Wilson was wrong about Gatsby; what matters is that honesty and lies brought about his destruction.
Quote on Jordan cheating