Gatsby's "colossal" dream, as Nick referred to it, is to literally repeat the past. Gatsby lives with the illusion that through the force of his will, he can negate the previous five years and that he and Daisy can go back and begin their relationship again. Acting within his illusion, he wants Daisy to tell Tom she not only does not love him now, she never loved him at all. This passage from Chapter 6 explains Gatsby's thinking:
After she had obliterated three years with that sentence [her marriage to Tom] they could decide upon the more practical measure to be taken. One of them was that, after she was free, they were to go back to Louisville and be married from her house--just as if it were five years ago.
When Nick tries to reason with Gatsby, telling him "You can't repeat the past," Gatsby is amazed and incredulous: "Why of course you can!"
Chapter 6 concerns Tom and Daisy’s first visit to Gatsby’s mansion, for one of his splendid parties. Daisy is impressed at the celebrities in attendance, but overall doesn’t have a very good time. “She was appalled by West Egg,” Fitzgerald writes, and despite her politeness it was very clear how she felt, for when the party was over Gatsby approaches Nick and says, with no prelude, “She didn’t like it.” He wanted desperately for Daisy to be impressed by his party, by his lavish guests and his lavish decorations; he wanted her to fall in love with his extravagance, and by extension be overwhelmed with admiration and love for him, for Gatsby himself. Nick states soon after this initial interchange that “He wanted nothing less of Daisy than that she should go to Tom and say: ‘I never loved you.’”
In Chapter 6, we see that Gatsby wants Daisy’s admiration, and he wants her love. He wants her to become dissatisfied with the life she has, he wants her to leave Tom and run to him. And he has lived with his illusions for so long he is surprised when these things do not happen immediately. “And she doesn’t understand,” he laments – “She used to be able to understand.” He is clinging desperately to a long-gone era of his life, so much so that he rejects Nick’s reality-check and says that of course you can repeat the past. Of course. As if it were the most logical thing in the world. As if it were the founding principle of the world. And he becomes overfull with the determination to recreate everything, to make Daisy fall in love with him by making everything as it was before – it is an obsession, and here we see the first fruits of that obsession, and the first real insight into Gatsby’s emotional disfigurement.
Jay Gatsby expects Daisy to renounce her marriage, discard the life that she has been sharing with Tom Buchanan, disclaim that she has ever loved Tom and forget about his (Jay's) five year absence and live with him as if all of this had never happened. He expects her to leave with him and continue their relationship, marry him as if he had never been gone. This is the measure of his dream.
Jay had been meticulously formatting his life to realize his dream - of being with Daisy. He had deliberately set out to raise a fortune, through fair means and foul, so that he could please Daisy and be the man that he thought she would be happy with. He had obviously realized that Daisy would only be reachable if he could measure up to Tom - be as wealthy, and thus he had completely immersed himself in achieving that ideal. Once he had reached that step, he believed that Daisy would forfeit everything to be with him, that she, just as much as he, would be prepared to 'repeat the past.'
Unfortunately, this was not to be. The dream was too immense. Jay's delusional fantasy was unreachable, for Daisy did not have the same ideal. She was shallow and materialistic, careless, and would not sacrifice anything. Jay was asking too much. And she told him just that.