Now that I know Nick, who is Gatsby?

After finishing chapter 1 of The Great Gatsby by F. Scott Fitzgerald, I still don't really know who Gatsby is.  The narrator keeps mentioning Gatsby, but he doesn't explain how he knows him or what he is like.  At the end of chapter two, I think Fitzgerald provides the first real example of Gatsby's personality or personal story.  He wrote: "...he stretched out his arms toward the dark water in a curious way, and, far as I was from him, I could have sworn he was trembling. (p. 21).   Until now, the story has only characterized Gatsby by mentioning his giant house.  Finally, we are given more insight into how he might play into the story.  Gatsby's actions here make it seem like he is longing for something.  Is he longing for meaning in a material-focused life?  Is there a person that he misses that is no longer in his life?  Furthermore, why is he looking out over the water, or is there even any significance at all?  If this book is anything like The House of Mirth, I would place my bets on the money/material theory, especially given the obvious riches represented by his house.  Based on what the narrator has said so far, he will get to know Gatsby more personally soon.  I'm looking forward to learning more about this elusive character.