And here's Gordon with no actual firearms; whatever he might've acquired from the Combine got dropped for lack of an equipment harness back at Abrams Island. Dang.

Well, there are rocks the size of a man's head up ahead. And the gravity gun has a pretty good grab range. Whatever's moving, if it isn't Terran in origin, is going to get smacked just as soon as he can get a clear line of sight on it.