Spider crack

I worked on boats as well and did gel repair.  We mixed a wax with the gel that would rise to the surface and that would seal it from air so it would dry.  And yes you do have to add catalyst.  You can also spray PVA over the top as well to seal it we just found the wax easier to use.  At any rate it is not an easy repair for the novice.

If you can get the pre-colored gelcoat from Forest River, you'd save yourself a lot of headaches in trying to match the color.  Wet gelcoat is dries darker, if I recall correctly from my sailboat repair days, so it's hard to anticipate the color shift from wet to dry.  Fading of the original gelcoat also has to be taken into account, even with the factory mixed color.  Boat yards and body shops that do fiberglass repair are likely your best bet if you want to get as close a match as possible.  Once you've prepared the cracks for gelcoat application, as others have mentioned, spraying polyvinyl alcohol over the wet gelcoat seals it from the air to allow curing.  It washes off with water.