Is there some physical decline? Absolutely, he's not as explosive as he was and he is more likely to get ouchy injuries than he used to be. KG does things differently than he used to, though, and it's hard to argue against the effectiveness. On Boston he isn't used to initiate the offense as he was in Minnesota, he is used as a finisher. Thus, his assist percentage has gone down but he's shooting (by far) career-bests from the field. On Boston he isn't used as the only defender AND the only rebounder, as he often was in Minnesota...he's allowed to specialize in specific roles here, consequently his rebounding percentage has gone down but his defensive rating has gone to the top of the league.
And just like in his prime, KG has still been at the top of the league in +/- since he got to Boston. And KG is also shooting almost 80% from the field during crunchtime this year...a percentage that he wasn't able to approach when he had to be Mr. Do Everything All Game for Minnesota.
So has he declined? Maybe a bit. But on the whole, he is still right at the top of the NBA so it doesn't really make a tangible difference.