The main conflict in The Great Gatsby is a simple and ancient one. Two males are fighting over one female. Gatsby is the protagonist because he is the one who initiates the conflict. Tom Buchanan is the antagonist because he is the one who is defending his home. Daisy is what in Hollywood would be called the MacGuffin. She is the "bone of contention." For a strong, lasting conflict to exist, there has to be something specific the conflict is about, and Daisy is what these two men are fighting about.
Tom is portrayed as rich but rather stupid. He is also unfaithful and has gotten emotionally entangled with another man's wife. This prevents Tom from realizing the threat that Gatsby represents, even after he meets him at one of his sumptuous parties. The real showdown doesn't occur until Chapter VII, when the two alpha males nearly come to blows.
"Your wife doesn't love you," said Gatsby. "She's never loved you. She loves me."
"You must be crazy!" exclaimed Tom automatically.
"She never loved you, do you hear?" he cried. "She only married you because I was poor and she was tired of waiting for me. It was a terrible mistake, but in her heart she never loved any one except me."
Tom is on the defensive because he has been habitually cheating on Daisy. Myrtle Wilson is only his latest mistress. But he has weapons to use against Gatsby, and this chapter is where he brings them out.
"I found out what your 'drug-stores' were." He turned to us and spoke rapidly. "He and this Wolfsheim bought up a lot of side-street drug-stores here and in Chicago and sold grain alcohol over the counter. That's one of his little stunts. I picked him for a bootlegger the first time I saw him, and I wasn't far wrong."
Gatsby has lost the conflict when Daisy understands that his wealth has all come from criminal enterprises. Gatsby's facial expression is described by Nick Carraway as like that of someone who had "killed a man."
. . . he began to talk excitedly to Daisy, denying everything, defending his name against accusations that had not been made.
Daisy begs her husband to stop quarreling. He relents and tells her to let Gatsby drive her home in his big, conspicuous yellow roadster.
"Go on. He won't annoy you. I think he realizes that his presumptuous little flirtation is over."
This contemptuous gesture by Tom, the victor in the conflict, leads to the tragedy. Daisy insists on driving the car and accidentally kills Myrtle Wilson, who has run out of the house thinking that the yellow car belongs to Tom Buchanan and wanting him to take her away from her husband who is holding her prisoner. Gatsby, characteristically, lets it be assumed that it was he, not Daisy, who was driving the big car. George Wilson learns that the yellow convertible belongs to Gatsby and assumes he was the hit-and-run driver. Wilson goes to Gatsby's house and shoots him while he is floating in his swimming pool and then kills himself with the gun.
The novel is complicated because there are many minor characters and sub-plots involved and because Gatsby's plan to win Daisy has so many ramifications in the past and present. But the essential conflict is that of two men fighting over a woman. Tom probably doesn't love Daisy but regards her as a possession. Whether Gatsby is in love with Daisy or with an illusion in which she plays a vital role is a question often asked by critics who may find it hard to understand why Gatsby should attach so much importance to this one woman.