Two passages are very relevant: the first is at the beginning where Nick says, 'If personality is an unbroken series of successful gestures, then there was something gorgeous about ..(Gatsby)'. On the surface Gatsby has achieved the American Dream: the boy from humble origins made good.But Gatsby hides his humble origins behind the image of the wealthy aristocrat with his Rolls Royce, his Oxford education and his extravagant parties. However, this persona is a sham, his wealth built on illegal activities wiith the likes of Meyer Wolfshiem. Also, not one of the people who attended Gatsby's parties attends his funeral; despite these wonderful events that he stages, it's as if he himself never existed, hardly a mark of a successful life.
The next passage is at the end where Nick stands on the lawn from which Gatsby would contemplate Daisy's green light: 'He had come a long way to this blue lawn, and his dream must have seemed so close that he could hardly fail to grasp it.' But he does fail, he does not succeed; in the beginning Nick also spoke of Gatsby having 'a romantic readiness such as I have never found in any other person'. He has invested his vision of Daisy with all of this romantic readiness and she has been quite unable to live up to it. It's doubtful whether anyone could have. So Gatsby is a magnificent failure, hence the 'Great' of the title.