"My dear Hamlet, stop wearing those black clothes... remembering your noble father..."
"I doubt it is no other but the main: His father's death and our o'erhasty marriage."
"What have I done, that thou darest wag thy tongue In noise so rude against me?
This scene is important because this is where you get to see who Gertrude really is. She almost admits to not being sad that her husband has been killed. She out right tries to defend her new husband (Hamlet's Uncle) and telling Hamlet to be nice to him. Gertrude also tells Hamlet to stop mourning over his father.
Gertrude is now acknowledging why Hamlet might be mad. She is noticing that this is all effecting Hamlet in a negative way. Her actions are becoming more loose in the sense that she only cares how it will affect her and the king. There is absolutely no mourning for her at this point.
Here Gertrude is saying "What have I done that you're talking to me so rudely?" She is trying to play dumb and acting like she hasn't done anything. Gertrude's attitude has changed to where she is almost playing along with Claudius. She is being defensive and trying to make Hamlet upset for yelling at her but it isn't working in the way she thought