I've been quite pleased with Starbrite's Polish with PTFE. Keeps the shine for quite a long time. BUT, if your gelcoat
is oxidized or hazy, you'll need to take that off, first. If not too bad, then almost any cleaner wax will do (Starbrite's is fine, I've been using 3M's and it does fine, too). If pretty oxidized, then you're looking at rubbing compound and there's no way around it.
So, it is a 3-step process. You've step 1 done. Now, you've got to get your gelcoat
nice and shiny. Then, you use the polish as the top coat and protectant.
It is a lot of work!