Never try to gel coat repair shallow scratches. A thin layer of gel coat isn't structurally strong enough to prevent itself cracking off. Either deepen the scratch using a small blade or grinder, or leave it. Unless the scratch goes through to the glass matting it won't harm the boat.
Thin hairline cracks - star cracks - present a bigger problem. These need to be repaired eventually. It's ok to leave them until the end of the season but much longer than that and water will penetrate the glass fibre matting and cause it to delaminate. These need to be widened using a small blade or grinder and then repaired with gel coat.
Wipe the repair with styrene to re-activate the old gel coat. This provides a chemical bond rather than just a physical one. Apply masking tape around the area. Add the catalyst to the gel coat according to the manufactures advice and mix well. Brush the gel coat over the area, remove the masking tape and leave it to dry thoroughly. Sand the repair flush using a sanding block and working your way through wet and dry paper, from a course grit to a fine one. For best results polish the area. For a bomb proof job, glass a thin layer of matting on the inside of the hull covering the area of star cracks.