I think KG is an all-time elite defender, with the likes of Hakeem and Russell. There is nothing he couldn't do. Athletic, fast, strong, vocal.
On offense, he was extremely versatile, and one of the best passing bigs of all time. He could post-up with ease because of his length, or take it outside with an excellent jumper with range.
The only PF I would rate ahead of him is Duncan, and only because 4 rings can't be ignored. However, one player came into a great situation (with a great mentor and coach) while the other fell into a mess of an organization (revolving door of coaches and lack of quality players).