They weren't using gravity, they were electromagnetic rails that shot the bombs out of the bay into the enemy ships. I mean it's still dumb since you could fire them from further away, just not dumb for the gravity reason.
I'm not a 100% sure I understand this. IIRC it was bombing spaceships hovering above a planet dropping bombs into the atmosphere, correct? (Totally possible that I'm misremembering!) In that case using gravity would be absolutely fine.
Oh, the bombs were meant for another ship, not the planets surface? Then indeed gravity makes little sense, or at least it's a very straightforward and uninspired port of the concept of bomber planes.
But I guess they wanted it to fit in with everything else... :(