As Grant has mentioned, it looks like the glass has been flexed and separated. That will require some further work to repair before gel-coating. It may be possible to make this work without removing the trim/rub-rail, but it will probably crack where the old meets the new gel. Fix it once correctly or be prepared to do it again.
Your steps are to:
- first remove any loose gel-coat and split/separated fibreglass. Feather the fibreglass back far enough to build up layers to the original. You may be able to use epoxy and fillers with small holes to rebond small separations in the glass, but new layers are better. Use a 1:10 rule for depth to width for feathering.
- use a similar rule for the gel-coat. It needs a good surface to cling to and it needs similar feathering or you will always see the repair. Colour matching will always be a challenge in a visible area. Ensure you read the directions for the gel-coat you are using to ensure a proper bond and cure. Plan on numerous layers and lots of sanding. There are a number of repair jobs posted on this site that are more involved than yours. Even I had one a few years ago involving hull damage from a rudder and prop hitting a rock.
Did I forget to say, lots of grinding, fibreglass/epoxy work, sanding, gel-coat, sanding, gel-coat, sanding, gel-coat, repeat... polish, worry about it, and finally forget about it as just a distant memory of a bad day on the water.