Thanks for your question. I would not recommend you use an additional essay to address your GMAT score.
For your GMAT, it's important to remember that on your MBA application your highest GMAT score is self-reported. For this reason, there isn't any harm in taking the test again, as admissions teams will focus on your best score. We normally recommend that applicants who believe they can improve their score by 30+ points take the exam again. MBA.com provides 2 free official GMAT exams and we also provide free GMAT tips here: http://www.stacyblackman.com/test-prep-hub/
If you'd like, we also provide free 30 minute phone consultations here: http://www.stacyblackman.com/contact/
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MBA blogger, US News and Author, The MBA Application Roadmap