Why is Gatsby so"great"

"He Smiled understandingly- much more than understandingly. It was one of those rare smiles with a quality of eternal reassurance in it, that you may come across four or five times in life. It faced- or seemed to face- the whole external world for an instant, and then concentrated on you with an irresistible prejudice in your favor. It understood you just as far as you wanted to be understood, believed in you as you would like to believe in yourself, and assured you that it had precisely the impression of you that, at your best, you hoped to convey." pg.32

During the course of the book I've been searching for the meaning behind "Great" which is before Gatsby's name. I never understood what could make a man so great or so enigmatic that one simply had to write a journal about their meetins like Carroway did. This description of him simply illustrated the quality in Gatsby that is so irresistible. It is the quality that is seen through nothing more insignificant than a smile. A smile that encompasses all the qualities one wishes to have and to convey. It is acceptance, understanding, and complete confidence in the person you are smiling at. Somehow Nick Carroway is able to see this in Gatsby. I think this instant is what captures Nick to Gatsby. He finds something foreign in Gatsby that has not presently been shown to him by others that have surrounded him in his society.