Well first your going to need the exact color Gel.

Because the the area is so shallow I would try just building the area up by spraying Gel and then sanding. It could also be done by adding a thickener to the gel and spreading it over the area then sanding.

If you haven't done any repairs with Gelcoat this may not be a good place to start. You might want to pratice on something else first or have someone that knows do it and hopefully you could be there to gain some experience.