Rubbing compound and a power buffer. 3M makes a compound that is specific for fiberglass gelcoat finishes, http://solutions.3m.com/wps/portal/3M/en…
It's best to use a foam diamond pad on the buffer, and use a low speed (100-150 rpm). You should also remove all the fittings from the area you are trying to restore, for easier buffing. After you have the oxidization removed, apply a couple of coats of UV protected automotive or marine wax to protect the finish.
As the gel coat finish does not suffer from oxidation which is a chemical reaction, can you describe the surface of the area concerned and the possible damage ?
Hopefully you are not suffering from Osmosis. If you are then the treatment which HB has suggested is not appropriate.
Osmosis is a serious condition and can take up to a year to clear and repair. If very bad then it may not be economically possible to repair.
it is very hard work and it does not last long for sure. I know because my boat suffered from it as well.
Skill and experience is needed in order to avoid lots of unnecessary waste of money, time and labour.
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Rubbing compound followed by waxing.