The hardest part of recoating gelcoat with anything, is getting the oil/wax out of the pores. Esp. if you were a good little sailer and waxed your gelcoat once in awhile. Gelcoat or any two part coating is very toxic and (too the best of my knowledge) has to be sprayed on. Hence can you say Ka-Ching!
We painted a 35' cruiser at the marina I worked in, with a two part coating to the tune of $6000..!!
Any of the high quality one part polyurathanes will adhere well if you get that old gelcoat clean and free of oil/wax. Something like 3 good coats should do the job (read the directions). I would wet sand between coats.
On the Cutter in Ket West, we used white Interlux with a dash of dark blue for whitening (blinding in the sun, but pretty). The only reason gelcoats go bad (asisde from ramming the dock and such) is from a lack of waxing. The sun is going to eat whatever it hits first and that might as well be inexpensive wax. So if you use the poly...WAX IT!!