In Chapter 5 of The Great Gatsby, what had Gatsby turned Daisy into in his own mind?

At the end of Chapter Five, Nick observes that Gatsby had turned Daisy into some kind of illusion. After spending their first afternoon together in five years, Nick says that for Gatsby, "There must have been moments...when Daisy tumbled short of his dreams---not through her own fault but because of the colossal vitality of his illusion." He adds, "No amount of fire or freshness can challenge what a man will store up hin his ghostly heart." At that point, Nick leaves the couple by themselves.