Gallas has been amazing for us since coming back from injury. He is great, really really great. But they had 2 big strikers, they are one of the top scorers if not the top scorers of headed goals. Their goals come from crosses, set pieces and things like that. Their team is based around wing play and crossing the ball. Kaboul got injured, Nelson came on for him. King didn't have a good day, he was troubled by Grant Holt... Now I love Gallas, but no matter what you say about Gallas, it's hard to call him tall. We saw at Stoke that he's not tall. (Some would say Harry might have noticed that before playing him at Stoke...) So while Gallas would have been useful on the ground, calmer while being pressed than some defenders, the thing you want playing against Norwich is being able to defend in the air.
It's true that if you look at retrospectively, Gallas for King doesn't seem like a bad idea. But at the start of the day, playing against an aerial team, you'd have backed King to help more than Gallas, wouldn't you?