I agree, even though this is hidden under the bumper it needs to be sealed back up. Exposed fiberglass is not a good thing on a boat.
I am not an expert fiberglass/gelcoat repair person, but have done some gelcoat repairs on my boat. From the looks of it, most of the damage appears to be chipped off gelcoat. Near the force of the hit it does look like some fiberglass has been damaged. You will most likely need some fiberglass repair and then reseal everything with gelcoat.
Since this is a hidden area and not structurally important, I might try to fix it myself after some fiberglass research. If you don't feel this is something you would like to experiment with then take it to a professional and be done with it.