On what constitutes a low GMAT: I would say anything below the 50% range of the school would be an actively low GMAT. So for HBS's case the scores are:
Middle 50% GMAT Score Range:700-760
Average: ~720.
So i think this is how it would break down:
<700: You've got an uphill battle as you'd be in the bottom quartile of the accepted class. Obviously not impossible, but you will probably need a high GPA or something else special to offset your gmat. I think adcom would describe this as "low".
700-720: Your GMAT isn't helping you, but as long as you have a decent GPA, it probably won't be much of a factor. I think adcom would describe this as "good"
730-760: GMAT might help you out a bit with a weaker GPA, but it isn't going to blow the adcom away. In this range, I think you're probably pretty well positioned. I think adcom would describe this as "solid".
770-800: GMAT is actively helping you. Obviously, the score won't make up for a 2.0 GPA or uninteresting work experience. But this will definitely be a positive factor in your application. I think adcom would describe this as "impressive".