If you're going to use a sealant you'll need to do more work on the fiberglass, which can be a nearly impossible without a machine. Then again you'll have to see if the gelcoat is just too far gone.
There are special gel coat restoration acrylics that will get the shine back, but then a year or so later it has to be stripped and reapplied.
Gel coat that is allowed to go to far in the beginning is really tough to bring back. Gel coat needs to be taken care of starting day one.
At this point I wouldn't bother sealing it unless you get it shinning like you want. Sealant will shine like wa and last about a year. Go with some colinite 845 in a liquid and see how that works for you. Colinite is a good protector..
Use it sparingly as a little goes a long way.
Tip: if you put wax, compound and it dries and is hard to get off. 1. You used too much of the product. 2. Spritz it with some quick detailer and it will come right off.