I believe the reason that Gatsby is decribed as 'Great' is because of his clear tenacity to have Daisy. He longs for her, so much so, that its clear he's in pain over his desperation to 'own' her. To want someone for so long and still be sane is a strength; therefore it is clear that Gatsby is indeed 'Great'! Not only that but also what he's willing to do to obtain her; for instance he became a bootlegger for his wealth and money and even brought a home in the exact location for a view over the water to her home; he even goes to the extent of having wild parties for people he doesnt even know in the hope that she will come to one. His whole life has evolved around Daisy, that is all he has ever wanted or needed and this book shows this dramatically.