First look at the gelcoat with a strong magnifying glass or pocket microscope (Radio Shack). If you see NO 'alligatoring', then the gelcoat can be wet sanded and powerbuffed back to the 'original' shine and brilliance.
1. wash the surface with a STRONG caustic detergent ... from a janatorial supply, then use oxalic acid (wood bleach -- from a paint store) to remove old dead wax, iron and tannin stains. Dont MIX detergent and oxalic !!!! rinse in between the two. Wear gloves when using oxalic as it quickly absorbs through you skin and can damage your kidneys.
2. Flat sand the gelcoat with 1000--> 1500 -->2000 grit wet and dry sandpaper using a few drops of dishwashing detergent in a few gallons of water. Use a rubber sanding block. Sand away until the surface is flat. Most gelcoat is quite thick .... but stop sanding immediately when you see the matting layer of fiberglass under layer showing through .... if you do its time to paint. If not, proceed to 2. The object is to remove the oxidized outter layer of gelcoat and produce a FLAT surface ... use the pocket microscope if you have one.
3. Get a high speed autobody power polisher (varibale speed 1000-3000 rpm), 3M foam polishing pads, Coarse Fiberglass rubbing compound, 1000 grit auto body shyop compound, 3M finesse it, 3M Perfect polishing compount, Collinite Fleetwax.
4. Begin with a fresh foam pad (Never mix pads and grits). In a 2ft. by 2 ft. surface apply coarse rubbing compound then do the 2ft. x 2 ft. square with a foam pad, move to the next 2X2 square until the whole boat is completed, Then do the same with 1300 grit - whole boat, then Finesse-it, then Perfect-it. Keep the buffer moving at all times, be very careful and dont 'burn through' sharp corners. Power buffing can remove a LOT of gelcoat, so use a 'light touch' .. and keep the buffer moving so you dont 'overheat' the surface. The speed of the polisher is what does the work not the pressure. By the time you get to the Finesse-it the hull should be back to NEW condition shine wise. Perfect-it with give the deep shine.
You must wax immediately to fill the 'pores' of the gelcoat. Use a clean bare hand, some water and the paste collinite and rub by hand until the wax begins to shine, then add about a 10˘ size spread over the 2X2 and powerbuff. Push the wax INTO the pores of gel ... dont just smear it across it - this will seal the pores and will retard oxidation.
99% of the time the above process will restore an old faded hull back to NEW condition. Its also the same process that is used to powerbuff a NEW hull when its pulled from its mold. Rewax at about 6 months and every 12 thereafter as a minimum. Every 2-3 years STRIP the old dead wax with a caustic detergent, light powerbuff with Perfect-it and rewax. .... This should keep the boat looking like new for the next 30 years.
You may want to fill in gouges, etc. with new gelcoat before you powerbuff.
Beware snake oil: acrylic coating 'wipe on' coatings. They look good for a season or two but ultimately fail and have to be removed (when they will look like a severe skin disease). The acrylic removal is a PITA and is ultimately more harmful than the above powerbuffing method.