]Gertrude is pretending to be a virtuous wife and concerned mother

Are you referring to "assume a virtue, if you have it not" and whatever Gertrude does to follow Hamlet's advice (if you could call it advice) after the scene in her chamber?

It's unclear to me whether she does refrain from sleeping with Claudius or does anything differently than she did before the confrontation. She tells Claudius that Hamlet is mad, which Hamlet tells her to do, but she appears to believe that he is mad, at least to some degree. He sees a ghost that she does not see, for example.

Various productions of the play have taken some or all of the ambiguity out of Gertrude's actions after her confrontation with Hamlet, but there isn't any actual text that tells us whether she does assume a virtue and pull away from Claudius or not.