I think that the most evident topic for the Gentlewoman to address is the mental decline of Lady Macbeth. It becomes really apparent as the play progresses that she is losing her grip on reality. Given what she was at the start of the drama to what she ends up becoming at the end of it, I think that it becomes essential for her to be "spoken" for by someone, anyone who is seeing her devolve into something so radically different from what she was.
Enter the Gentlewoman. She could speak in Act V as to how her "lady" is literally "losing it." It might be effective for someone like her to comment because there is no one else who is going to be speaking for her. Her husband, Macbeth, has "other things" on his mind and really the servants would have to speak for her. I think that articulating the sense of helplessness both the Gentlewoman and Lady Macbeth display would be a suitable topic. In this, the Gentlewoman would be able to speak the words that the audience feels at such an entropy of character. The tone could be one of remorse, helplessness, or even a sense of justice being established in a world where there is so little righteousness in the face of wrong. I would probably have the Gentlewoman speak these lines before news reaches to her husband of his wife's death so that the audience can see and contrast both reactions to the death of Lady Macbeth.
thanks so much!! that is so helpful! and a very nice twist on my original thought that really made my day thanks! :)