These are cracks created by impact to the surface and not scratches from movement along the surface?
If they are similar to scratches, opening up scratches for gelcoat is addressed 1 minute 50 seconds into this video. One suggestion looks like a can opener. The other suggestion - a Dremel -- looks like it has a Dremel silicon carbide grinding stone accessory. Iíve seen a similar Dremel accessory used by Andy Miller in his Boatworks Today videos.
How to repair Gelcoat Part 1 by TAP Plastics
I've got several tungsten carbide Dremel carving accessories for the preparation Iím doing for gelcoat repair, but Iím not repairing damage that is hairline. So far, Iíve used one of the structured tooth tungsten carbide accessories, which Dremel lists for fiberglass, but the structured tooth accessories might be too aggressive for opening up something that is hairline. I havenít tried it. You would have to be extremely careful.