In my opinion, Gatsby loses Daisy because Tom starts to talk about how Gatsby got his money. As Tom tells Daisy that Gatsby got his money through illegal means, Daisy seems to lose interest in Gatsby to some extent.
I think that the best thing that Gatsby could have done would have been to point out that Tom is morally corrupt as well. To talk about how he is cheating on Daisy.
I do not know if Gatsby actually knew this, though. And even if he did, he might not have wanted to hurt Daisy like that.
Essentially, in The Great Gatsby, Gatsby loses Daisy because he is trying to recapture something that never existed in the first place. He can't recreate the relationship because it never existed in actuality as it does in Gatsby's mind. In short, Daisy never loved Gatsby like Gatsby loves Daisy.
Daisy won't announce to those present that she nover loved Tom. She tells Gatsby that he asks too much. It's not enough that she loved Gatsby once, or even that she loves him now. Gatsby has to have the whole illusion, and Daisy won't give it to him. Gatsby is doomed, period. Nothing he says could make any difference. He can't recapture what never existed.
Gatsby doesn't really lose Daisy becuase he never really has her to begin with. He doesn't even know her. He is only in love with an image -- he is not interested in knowing the real Daisy. Thus, he orders her around throughout the hotel scene in much the same way that Tom does. In the end, Daisy stays with Tom because deep down, there is a part of her that loves Tom -- that and the fact that he manhandles her once more. Bottom line, Daisy is weak; she doesn't have what it takes to stand up to Tom. Nevertheless, she doesn't belong with Gatsby either; Gatsby treats her much the same as Tom does.