Duncan might have been the more successful player, but KG is the better all around basketball player.
I would take KG anyday over Tim Duncan.
If you were to build a basketball player from scratch, would it go something along these lines?:
Tall, strong, quick and agile, can shoot the ball, can handle and pass the ball. A great team player, and plays excellent defence, ability to guard all positions (at least better than what others can).