He doesn't run with the body lean of a RB, he moves like a big WR. His running style is more conducive to the screen game
The same things can be said about McFadden. Gray runs high, but he runs strong and breaks tackles.
Gray had a good QB last year (Akeem Jones) and he wasn't overly productive at receiver. His big plays were either read-options or catch-and-runs. He can play receiver, but he is most comfortable with the ball in his hands.
Look at the catch at 1:45. A guy like Anquan Boldin attacks that ball. Boldin was Mossing guys as soon as he switched to receiver. I don't see those instincts in Gray.
http://www.youtube.com/watch?v=5kySAuuCa4U