Most times, oxidized gelcoat can be brought back, it’s just a matter of how much labor will be involved, particularly on lighter colors. Most boats will have sufficient thickness of gelcoat so it’s a matter of working with the right abrasives to cut thru the “bad” layers and start to polish out the scratches that are left.
Most times, a heavy compound like the 3M mentioned above with a good wool pad will be a good starting point. In some cases, I’ve had to resort to wet sanding to save some time, as the cut wasn’t enough with even a heavy compound.
For a 30-ish hull, that’s leaving chalk on your fingers, I would plan on at least several man-days of work