In order to construct a monologue for the Gentlewoman, Lady Macbeth's lady-in-waiting, you would need to pick out her most important contributions to the scene and then have her give voice to the fears or concerns that motivate them.
First, she is clearly disturbed by Lady Macbeth's sleepwalking, so much so that she has called the doctor to come and wait up with her so that he, too, can see the queen's strange behavior. When he asks her what she has heard Lady Macbeth say, she replies, "That, sir, which I will not report after / her" because there is "no / witness to confirm my speech" (5.1.15-16, 5.1.19-20). She seems to fear having her used words against her, perhaps because life in the Macbeth household at this point is so full of paranoia and suspicion.
Next, the gentlewoman tells the doctor, "It is an accustomed action with her to / seem thus washing her hands. I have known her / continue in this a quarter of an hour" (5.1.30-32). She knows that this action is linked to some terrible guilt resting heavily on her mistress's conscience because she says, "I would not have such a heart in my / bosom for the dignity of the whole body" (5.1.57-58). In other words, she recognizes that Lady Macbeth's malady is emotional, even perhaps spiritual, in nature. This would likewise make her fear to expose her mistress to the ridicule or judgment of others.
The gentlewoman truly seems to wish that all will be well, and when the doctor says so, she responds, "Pray God it be, sir" (5.1.61). She appears to pity her mistress. Even though the queen clearly has some terrible deeds on her conscience (so many that she imagines that she cannot clean the blood from her hands), the gentlewoman seems to have no wish to betray her confidence. That, or she is so scared to betray her mistress that she can only call a physician (that way, she can say that she was only concerned about Lady Macbeth's health). In constructing a monologue for this character, it might be important to emphasize either her pity or her fear as the motivation for calling the doctor, or to explain how both factor, if you believe they do.