Well, that is a very odd, very rare resolution. In fact, so rare I've never seen it before. You might have to set the resolution in the ini (in the same folder you installed glest to, titled 'glest.ini'. Open it in notepad or your favorite plain text editor).
Look for lines with screen height and screen width, and change the height to be 720 and the width to be 1280. Hopefully that will fix it.
If not, talk to me again. If too complex to understand what to do, tell me and I'll dumb it down.