Moral decay/decadence, dishonesty/illusion, and dreaming are each major themes of this novel.
The wealthy class depicted in the novel demonstrates a repeated neglect of morality and of any feelings of sympathy.
The wealthy class is morally corrupt in The Great Gatsby...There are no spiritual values in a place where money reigns...(eNotes)
Far from paying a price for wrong-doing (with their various affairs, the killing of Myrtle Wilson, the abuse of Myrtle and George, the criminal behavior they may carry out, etc.) the wealthy class seems to presume that money has replaced all moral currency and all honor.
Duplicity and deceit are rampant as well in the novel. Jordan Baker is a cheater in her sport. Gatsby is a made-up person, essentially. Tom and Daisy each have affairs. There is also a falseness of self-presentation that adds to the duplicity of behavior.
Despite the moral failings and tendencies to lie found among the people depicted in the novel, they all maintain an ability to dream. This is especially true of Gatsby who, to the end, never loses his penchant for believing in the dream image he has set before himself.
"Gatsby believed in the green light, the orgastic future that year by year recedes before us.… Tomorrow we will run faster, stretch out our arms farther.… And one fine morning—"