The Gentlewoman makes a brief appearance in act V, scene I, and is Lady Macbeth's lady-in-waiting. In order to write a monologue for her, it is necessary to really understand her character, so that you can find her inner voice and be able to able to accurately express her thoughts and feelings.

To begin, the Gentlewoman is clearly very concerned about Lady Macbeth. For example, she tells the Doctor about Lady Macbeth's unusual sleepwalking activities. This concern leads the Gentlewoman to worry about what secrets Lady Macbeth is keeping:

Heaven knows what she has known.

Moreover, after seeing Lady Macbeth's distress, the Gentlewoman says that she does not envy her mistress's position:

I would not have such a heart in my bosom for the dignity of the whole body.

What we find, then, is that the Gentlewoman is both concerned for Lady Macbeth and afraid of what has prompted these episodes of sleepwalking. Your monologue should reflect both of these feelings in order to accurately convey the Gentlewoman's character. It might also be useful to use some or all of these relevant quotes as a basis for your monologue because this will help to create a sense of authenticity.

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.

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.

The Gentlewoman in Act V speaks in prose, not poetry, so the monologue need not be adorned with poetic language or conform to meter or sound devices.

Some insights into the Gentlewoman's character:

• She has been the solitary witness to Lady M's behavior.  She is lonely and needs to cooroborate the behavior so she can alley her fears.

Neither to you nor any one; having no witness to
confirm my speech.

• She associates Lady M's sleepwalking with letter writing.

I have seen
her rise from her bed, throw her night-gown upon
her, unlock her closet, take forth paper, fold it,
write upon't, read it, afterwards seal it, and again
return to bed; yet all this while in a most fast sleep.

• She is religious; she uses heavenly imagery.

She has spoke what she should not, I am sure of that: heaven knows what she has known."

• She focuses on the body.  She uses body/mind/soul imagery:

I would not have such a heart in my bosom for the
dignity of the whole body.

• She focuses almost entirely on Lady Macbeth, not herself; she seems very faithful in her duty toward her majesty