The essential plot of The Great Gatsby is very basic and simple. Two men are fighting over one woman. The man who initiates the conflict is Gatsby. He is the protagonist. Without him there wouldn't be any conflict or any story. The other man has to be Daisy's husband Tom Buchanan. He is the antagonist. Gatsby manages to have a love affair with Daisy practically under her husband's nose. The conflict really doesn't break out into the open until Chapter VII when Tom asks:
"What kind of a row are you trying to cause in my house anyhow?"
Daisy is the "bone of contention" or "MacGuffin" in this conflict. She feels herself being pulled in two different directions. She tries to intervene:
"He isn't causing a row," Daisy looked desperately from one to the other. "You're causing a row. Please have a little self-control."
"Self-control!" repeated Tom incredulously. "I suppose the latest thing is to sit back and let Mr. Nobody from Nowhere make love to your wife."
In the end it is the antagonist, Tom, who wins the struggle. He has several advantages over Gatsby. Tom, after all, is already married to Daisy. Tom represents "old money." He has also found out a lot about Gatsby's criminal activities and gangster associates. Daisy is a weak character. She stays with her husband and her little daughter even though she no longer loves Tom and does love Gatsby.
A rather weak point in the plot is Tom's consenting to let Daisy and Gatsby drive back to Long Island together in Gatsby's big roadster. But F. Scott Fitzgerald evidently wanted the story to end with Gatsby's death. His death, like the death of Anna Karenina in Tolstoy's novel, makes the story a tragedy and closes the novel with finality. Gatsby is totally defeated. Otherwise, he would have gone on loving Daisy and probably trying to win her away from her husband.