I find it odd that we've never discussed this one (I had to search my archives to make sure). In the context of Hamlet, do you think that Gertrude knew what Claudius did, or not? Was she in on it? When did the relationship with Claudius start - before or after the death of her husband?
I've always taken the position that she "knows", but she's in a state of shock and denial about it. If she ever stopped to think about it she'd have to admit what she knew to be true, so she gets around that by never thinking about it. She shows no guilt, like Claudius does ("My offense is rank...."), so it's unlikely that she is consciously aware of just how bad her actions are. That's why in the bedroom scene her line "As kill a king?" is not so much an "Oh no, Hamlet knows!" moment, but rather the first time that she actually has to consider the reality of what she's been a part of.
But then...if that's true, and she realizes that Claudius is a murderer, she sure doesn't seem to upset by it in the later scenes. So maybe I haven't thought this through.
Anybody else? As I write this I wonder if perhaps she does know right along, and really does have no guilt about it, because it was Claudius she loved, so she's happy to have her husband out of the way.