In a way, the title is satirical or ironic. Gatsby is great because he is larger than life. He is a legend of himself. He is not so much a man as a cartoon. He is like a comic book superhero. So he is the Great Gatsby.
I agree that Gatsby is great! Gatsby has a dream-Daisy. He also has a generous spirit. He lets people eat and drink to their hearts content on his dime, and yes-he throws the parties in the hope one day Daisy will show up-but he lets strangers enjoy themselves time and time again instead of weeding out those who would not help his cause. He thinks things are what are important to Daisy, but they really aren't to him except in their value to win Daisy.
He also never stops his worship of Daisy-that's dedication!
Good question! It's a good one for discussion because it can also be asked: is Gatsby "great" at all?
I'll kick things off by saying Gatsby is great, especially within the time he lives, for making something of himself. He creates a new identity, becomes fabulously wealthy, and woos the woman he loves. He invents his own reality, which is a 'great' feat-- especially in a world where image and reality are at times indistinguishable.
Gastby is great because he is a goal driven man who worked from the bottom up to get to where he is now. From nothing, he was able to create an identity for himself in a world where status and money are very important.
Also, Gatsby is depicted as young, rich, and romantic. What more can a man need?
Gatsby is an example of an American dream (even though he may have been involved in illegal bootlegging) where he has almost everything a man desires. But the irony of all this is that money cannot buy happiness within a society that is basically superfical and concerned with staus.