The novel does not make direct moral statements about the characters' lives, nor does it explore religious themes. The reader, however, can infer from the characters, settings, and events that moral values have been replaced by materialism--the desire for material possessions. Specifically, this can be seen in Gatsby: how he lives and who he had once been.
As a boy growing up in poverty, Jimmy Gatz had dreamed of being wealthy; he believed money would give him a meaningful life, one of beauty and romance. He created an image of the man he wanted to become and chose a new name for himself--Jay Gatsby. Nick recounts these facts about Gatsby's values:
I suppose he'd had the name [Jay Gatsby] ready for a long time, even then. His parents were shiftless and unsuccessful farm people--his imagination had never really accepted them as his parents at all. The truth was that Jay Gatsby, of West Egg, Long Island, sprang from his Platonic conception of himself. He was a son of God--a phrase which, if it means anything, means just that--and he must be about His Father's Business, the service of a vast, vulgar and meretricious beauty. So he invented just the sort of Jay Gatsby that a seventeen year old boy would be likely to invent, and to this conception he was faithful to the end.
The passage reveals a great deal and suggests even more. Gatsby does not embrace moral values; he does not seek God. Instead, he seeks beauty, "vast" and "vulgar," and spends his life being "faithful" to it. When Daisy Buchanan becomes the personification of Gatsby's values, he acquires vast sums of money, an enormous mansion, fine cars, fine clothes, and other material possessions to win her. Daisy becomes Gatsby's religion, and he worships her from the time they meet until he dies.
Immorality runs rampant in the novel. Gatsby's money comes from his crimes. Tom is a serial adulterer. Gatsby, Daisy, and Myrtle engage in adulterous affairs, as well. It is through George Wilson, Myrtle's deceived husband, that Fitzgerald again references God in the novel. George tells his unfaithful wife:
. . . God knows what you've been doing, everything you've been doing. You may fool me but you can't fool God!
Throughout the novel, George Wilson is the only character who seems aware of the presence of God; the rest of Fitzgerald's characters, surrounded by material possessions and living lives without moral value, have directed their attention elsewhere.
Really interesting question.
Using Gatsby as an example, we have to assume that he was once religious or contemplated God as a motivation in his life at one time. Since we don’t really know this, we could also say that a replacement for God is synonymous with something he worshipped, something he felt was the spirit and instigator of all creation. The simplest answer is Daisy, or love, or an embellished nostalgia for the past. By changing his name and identity, Gatsby himself acts Godlike in creating Gatsby in his own (made up) image. The green light is like an altar upon which he worships (gazes). In a larger sense, Gatsby’s entire quest to win her back is frequently called a prime example of the problematic illusiveness and promise of the American Dream. So, maybe it is the American Dream that replaces God. Daisy is like a Holy Grail. The American Dream is the possibility of claiming the grail. This is not to say that the American Dream is a corruption of God. Actually, the American Dream is a paradox. On the moral side, it is the promise that anyone, regardless of class, race, gender, etc. can rise up and be successful. But for the Dream to live up to its own morality, it must be obtained via moral means and the success obtained cannot be superficial. That is a overly moralistic view of the American Dream, but Gatsby employed unsavory and selfish means to achieve his dream and this was the corrupting factor.
This question certainly depends on what your concept of God means. If God determines all actions, and we have no free will, then what does this for Gatsby? Maybe it is his infatuation with winning Daisy back – not so much Daisy herself or for love, but the triumph of winning her back. Just an idea. If God is the creator of everything, but then sits back and lets us use our free will and choose our own fates, then maybe Gatz replaces God with Gatsby. He literally creates himself and puts this new self on Earth to save Gatz from heartbreak.