Gatsby is a man who is able to rise above his very humble beginnings, leaving his past behind to recreate himself. He is smart and willing to work hard and take risks to get what he wants. It seems that he has been successful until he falls for Daisy. His inability to let Daisy "go" when they meet the second time in his life is Gatsby's undoing. "Great" is a relative term. In some ways I think he is great in becoming someone much different than he would have been had he not had the dreams and desires to grow and be successful. However, like a tragic hero, he has a flaw that keeps him from greatness or true success. His ill-conceived love controls him and obsesses him. It is here, then, where he loses sight of all he wanted to be (though we may question the methods and means he used to get there), and then loses everything else that might have mattered to him. He loses himself, and this is truly tragic.
Given another go at titling this novel, I might suggest "The Reality Delusion" or "The Glamour Never Fades".
This book's central idea, for me, is related to identity. How do we see ourselves and how does this self-vision make us who we are, in our eyes and in the eyes of others?
Both titles I suggest here connect with an idea that the unreal can become real and/or vice versa, and, either way, the effect of the trick is lasting.
All the previous answers are really good and thought-provoking. To me, it always seems strange that Gatsby would fixate on a person as shallow as Daisy. The Great Gatsby is a good title for all the reasons suggested above, but, if one wanted to leave absolutely no mystery or suggestiveness in the title, one might have called the book The Henpecked Gatsby or Gatsby the Obsessed.
I would argue that he is only great in the way that he has fooled most of the society of the Eggs and he is great at bringing out the hedonism and hypocrisy of the people that flock to his parties.
I would call it the Less Effective Gatsby. He can't quite see through Daisy for what she is, he still has this idealistic vision of her that he can't stop chasing. He doesn't plan ahead enough to avoid dangerous and destructive situations and he really reminds me of lots of guys when they get dumped thinking they can be friends with the girls that dumped them.
Interestingly, the Great Gatsby wasn't Fitzgerald's orginal title. He originally planned to incorporate the name Trimalchio, who is a social climber and hedonist from the classical poem Satyricon. Trimalchio is really a contemptible character, and that Fitzgerald would contemplate using his name as the title of the book with reference to Gatsby shows that he didn't consider Gatsby great at all. The "Great" in the title is ironic.
Trimalchio is mentioned briefly in chapter 7, and an early draft version of the novel written with that title has been published in a Fitzgerald collection by the Cambridge University Press.
You can think of it as metaphorical. Gatsby is great in terms of his reputation. He is larger than life. Gatsby lives in his own world, where the reality the rest of us live in doesn't exist. He is his own legend.
I agree that he's not great. I think that all he is doing is trying to be something that he's not. He's trying to do so because he thinks it will make people (specifically Daisy) think more of him. So maybe it could be The Peer-Pressured Gatsby or The Insecure Gatsby. These are of course pretty lame titles, but they reflect more of what he really is.
In my opinion, no, he's not great. I see the title as reflecting what he wanted to think of himself as being - the image he had cultivated and was trying to present to the rest of the world. If I had to rename the novel with the same structure but a more accurate (in my opinion) adjective, I would probably call it The Delusional Gatsby or The Wishful Gatsby - neither of which have the appeal of the alliteration in the original title!
Gatsby is great because he was able to go from rags to riches. And he had the potential to go much farther, there was a quote somewhere about how he could have climbed the later to heaven or something like that but he could only do it alone and that Daisy was the reason that he couldnt reach that. And Gatsby was never really in love with Daisy herself, she was shallow and materialistic. He was obsessed with the image of her that he had created in his mind during all of the years they were apart.