OK, that's good news that the hatch is structurally sound. That means this is purely a cosmetic thing... with one caveat. Bare fiberglass can wick moisture in - one of gelcoat's "jobs" is to protect the fiberglass from that. So, from a functional standpoint, the only thing you "need" to do is protect the glass. How far you want to take this, aesthetically, is up to you. But I can tell you that trying to match the non-skid and color is going to be very time consuming. You also are dealing with, like any lovingly-used boat, non-skid that is worn down - so if you tried to match the non-skid with a mold, it won't since the new part would look... well... new.
Since forums are well known for "opinions", I'll give you mine as if this were my boat. I would grind out any loose or suspect areas, including that crack, leaving the exposed edge of the gelcoat at a shallow angle. Then I'd fill it with something like MarineTex or or PC-11. They are a "whitish" color that is pretty close to the gelcoat. They are structurally MORE than what is needed, but there is one downside and that's that they are not UV stable. All that means is that you need to topcoat it with a paint - even a clear poly would do it.
Another relatively easy "fix" is to (after grinding/sanding) coat everything with epoxy or polyester resin and not worry about trying to fill it in. But you'll need to topcoat that, as well.
