After Jordan tells Nick the story of Daisy, Nick says that Gatsby "came alive to me, delivered suddenly from the womb of his purposeless splendor. "
How does the metphor of birth help explain what Gatsby's behavior had meant to Nick up to then?
I think that what is going on here is that Nick Carraway had not seen Gatsby as a real person before then. After Jordan tells Nick about Daisy, Gatsby is "born" in his eyes -- he becomes a real person with a purpose.
Up until then, Gatsby's life had seemed pretty pointless to Nick. Why have the big house for just him? Why have all these parties when he doesn't even like them and doesn't really enjoy the people? It seemed meaningless and unreal.
Now that he finds out it's all for Daisy, it makes sense and Gatsby can be a real person.