I don't think you can make the grav gun a balanced, interesting weapon. It's inherently ammoless, so it can't be consistently powerful (or it would be used forever and destroy everything like in the final level). It can't be weak, because physics objects are going to be overpowered in every game (you can't justify throwing a giant saw blade or a toilet or w/e at a human and having it just bounce off, it's gotta be a one hit kill). All it can be is very situationally overpowered, i.e. once in a while you notice some stuff that can be used to trivialize an encounter.
There was one good in-combat usage of the grav gun that felt appropriate, required an amount of attention and skill, and wasn't overpowered. That was using it to throw grenades back at the combine. Now that I think about it you could probably do the same with their bouncing balls of get fucked, though I don't think they advertise firing that like they do throwing grenades and the speed would make it almost impossible to time.