The biggest knock on KG is that he fades from contact. He could score 30 ppg if he didn't shy from contact under the basket so much.
One advantage to this, however, is that his body might hold up longer. Plus, a big part of his game is shooting jumpers, fade aways, and finesse plays. Those don't really rely on athleticism.
He is starting to lose a step on defense though. But he'll still be fully capable of putting up 20 and 10 into his mid thirties.