I have done a fair amount of research
on this subject since I am going to do it myself. Check out Maine
Sail's post under the "Gel Coat Restoration" topic in "Construction, Maint and Refit" section. He's posted some pictures of his work and they're good enough for me. Here's his quote:
"The Cliff Note Version:
#1-Clean the hull
with an acid base cleaner like FSR or On & Off to remove tannin staining.
#2-Wet Sand by hand 600 then move up the grits to P1000 (only if severely oxidized other wise start below)
Super Duty Rubbing Compound (use a wool 3M super buff COMPOUND grade pad)
#4-3M Finesse It (Use a foam 3M #05725 pad)
#5-(OPTIONAL STEP) Meguiars #9 Swirl Remover (professional grade automotive product tan bottle - Use 3M #05725 pad)
#6-Collinite #885 Fleet Wax Paste Version- or any top quality carnuba paste wax"
The people I read advise staying away from silicones, teflons and other synthetics as they are hard to remove down the line. I got most of my information from the article mentioned above as well as several auto detailing sites (and they are working with substrates much thinner than gel coat).
Since you'll probably shell out $20-30/ft (or more), I would have a detailed discussion with the yard on what's going to be done.