Its tough to explain away a low GMAT, unlike a GPA its a reflection of your "abilities" at the time of the application. You also have the ability to retake it numerous times and they almost always only care about your highest score. Unlike undergrad where you can explain it away because of the need to work fulltime to pay for school, and numerous other ways to explain some bad grades...the GMAT is hard to give a believable reason for.
I would think long and hard about what your reasons are before you try to write an optional essay about it. Where do you fall on the schools 80% range? Any reason you don't want to retake it? The GMAT is one thing you can improve and it really will strengthen your chances if you are below the average. There have been members of this board who have been waitlisted and told they will get in if they can increase their GMATs. Dont give them a reason to deny you if you can improve it ahead of time.