3.1 is borderline -- it's not bad that it becomes a huge concern, unless you failed courses. If your grades royally sucked in college, then yes you should at least acknowledge it in the optional essay - but it's literally an acknowledgement and not a dissertation with a list of ironclad excuses (in other words, there's only so much you can say -- beyond a few sentences, the readers' eyes start to roll).
As for GMAT, yes shoot for at least 700. If it's below 700, don't just give up because you're doomed - just know that you have a big hill to climb and you'll need a bit of luck on your side.
Alex Chu
alex@mbaapply.com
www.mbaapply.com
http://mbaapply.blogspot.com