I sincerely doubt the corrosion has anything to do with the flex gas line or missing bushing. Unless the photo angle is deceiving, that corrosion isn't even beneath the gas line; it seems to be a good 1-2 inches to the left of it.
Water should NOT be coming out of that hole. If it is then you have bigger problems than a missing bushing.
The water is likely dripping from the ceiling, or at least it was at one point.
Anyways, the missing bushing is important and a sign of bad workmanship. If the flex hose is disturbed and rubs against the sharp metal then it could cause a gas leak.
The installer should have used black iron inside the furnace and made the flex connection from the outside. See below for a picture of how it should look.
So yes, the missing bushing is an issue but not in the way you guessed.