You don't need to explain your sub 700 GMAT score, especially since you have good grades in college.
I agree with dock13333. The optional essay should ONLY be used to address 1) significant weakness in your application or 2) significant strength that somehow isn't in your essays already.
You need to look at it from the AdCom's perspective. They read thousands of essays, the last thing they need is to read an essay that does not add significant value to your candidacy. A poorly used optional essay will make them question your judgement If everyone wrote an optional essay, it would create much more work on them.
Hey there. Good question. You don't need to write an essay about your GMAT unless it’s really really low (we are talking about 10th percentile or lower). If you have a 670, or even 650 I wouldn’t write a whole essay about it. Plus, really, what can you write? All they will think is that “maybe you should have done better”. So instead of that, don't worry a bout your GMAT. Your score is your score. And now beat all those OTHER essays!