Honestly, I'm not so sure if Gabriel will turn evil again. Yes, it's possible, but, in the future episode, he wasn't And yes. I know, the future can be or is already changed, but we still saw, there were things, that didn't change, no matter what future we saw.
What I'm really wondering about is more the point: Why does Arthur take Peter's abilities, like he wasn't his son und "keep" Gabriel at his side. I mean, he knows gabriel did save Peter and that Gabriel wants to be a good person. There has to be a reason why Gabriel and not Peter. And if it's right, that Gabriel just "plays" to be on Arthur's side, than Arthur knows that also. So why keeping this son so close then while getting rid of the other son?
Edit: I totally forgot to say... I like the new Sylar/Gabriel. He is good but still feels like "you don't know everything about him and he surprised you so many times so he can do so again"