If its that bad, don't waste your money on so-called restorers - paint it. I did and it was not difficult and lasted well. Find a handy booklet printed by International Paints for easy instructions.Rab.
First wash to remove dirt salt etc. Then use a fibre glass rubbing compound, there are several on the market, on a compounding mop fitted to a polishing machine or an electric drill running at slow speed. Keep it moist, I keep a wshing up liquid bottle full of water handy and work on a small section at a time. If the gell coat is very bad then start with a heavy duty compound. Allow to dry and remove with a soft dry cloth, or clean off wih a wet sponge. Then go over the whole thing again with a good quality fibre glass polish and a polishing mop on the machine. You should be able to get the compouding mop from a car paint suppliers. Reckon to spend at least 2 days hard work on a 20 footer. If you repeat the process but more quickly every few years and you can keep a boat looking smart and put off the day when you have to resort to painting for a good while. Mine is red and is just about to be lauched for the 24th season looking almost as good as new. There is an article in a recent PBO but using a slightly different technique, but the above is based on article in PBO about 15 years ago.
I agree with the suggestion to polish. A painted hull is always at a disadvantage to the original. Cutting and polishing well is V hard work though. Try an area and if you feel it is too much work or the colour is not coming back then paint!
Farecla no 10 has worked extremely well for me - I have used it most years and still have over half a bottle left. Costs about £12 from good car accessory shops. I still consider the best polish I have used to be Mer, again about £10 or so for a litre. Using a sponge attachment for the Farecla on an electric drill at low speed I have completed the whole process on a 22' hull in an afternoon. The key is to keep the sponge attachment very damp. I then used a lambswool bonnet again on an electric drill to polish the hull. Make sure the bonnet does not become clogged with dust. This is one of the most satisfying jobs i have done on my old boat as you can see the improvement in looks almost immediately and for very little cost - i reckon the actual materials used each time cost less than a fiver! My gel coat went from looking like a very white cloudy blue sky to almost clear blue after the first effort and actually looked even better each time I carried out this process.