Gatsby's dream surpassed just winning Daisy back. He wanted to repeat the past with her. He wanted Daisy to tell him she had never loved Tom and to wipe out the years that had intervened since he and Daisy had been together in Louisville before World War I took him away. Gatsby wanted them to return to Louisville and to be married "out of her house" [her father's house and her childhood home]. When Nick tells Gatsby that he can't repeat the past, that nobody can, Gatsby is blindsided by the very idea. His entire world has centered on that premise.
Specifically, Gatsby's plan to win Daisy back--repeating the past aside--fails because he fails to recognize the power of the stark difference in their two social classes. Daisy is an "insider," born to wealth and comfort and married into the elite upper class of American society. She and Tom are far more alike in this respect than she and Gatsby could ever be. As Daisy herself says, "Rich girls don't marry poor boys." They also don't marry boys who used to be poor, who grew up outside her social circle, and who will forever be "outsiders" because of it. Gatsby did not understand this difference between them. He did not recognize Daisy for the shallow, selfish woman she really was.