He is playing more minutes. He can't just save energy to finish games anymore, especially since his defensive expectations are higher. He is playing more minutes, and expected to spend more energy in other aspects of the game other than scoring. Plus he isn't a surprise anymore, teams know they have to give him attention. A combination of things has limited his 4th quarter effectiveness this year.
I'm still undecided about Gordon. He has unreal skills, but I just wish he was 4 inches taller. I think his height will always be a crutch, unless he can find himself in a situation playing with a point forward type player. He would be great in the Derek Fisher role, or the Ron Harper role. Obviously I'm not speaking of calibur with those comparisons, but he needs to be a scoring guard, where his ball handling duties are light, and where he isn't matched up with opposing shooting guards.