When you're talking about Carnage, you're talking about someone who is completely insane like the Joker... what has ever given any indication that Gosling can pull something off like that?
I admit I don't watch a lot of Gosling movies, I don't seek him out, but from what I have seen, he doesn't have much charisma.
Jim Carrey isn't an awful choice, just a cheesy one, but that seems to be the route the Amazing franchise is going anyway.
Everyone remembers his B-grade Joker in Batman Forever. We don't want that again. What about... Jackie Earle Haley?