On one of the sailing forums, someone asked about what gelcoat cracks mean. This is basically the response I gave them.
Basically, there are THREE types of cracks you see in gelcoat.
1) Crazing or spiderwebbing—called this because the cracks tend to go in all directions and have no real central point or directionality. These are usually caused by age or by having the gelcoat laid too thickly.
Fairing over these with thickened epoxy is generally all that is needed, since there is actually no structural issue with this type of cracking generally.
2) Stress cracks—usually due to the laminate flexing under load and generally show up as parallel or concentric lines
Generally, this means that the laminate is too weak at this point and should be reinforced in some way. Adding stringers or floors to a hull or glassing in reinforcing ribs to the underside of a deck or even just adding more fiberglass—though this is generally the least useful—will help prevent the laminate from flexing further and fatiguing at this point.
3) Impact damage—usually due to something hitting the laminate and causing the damage and generally shows up as radiating lines or concentric circles with a common central point—where the impact actually occurred.
In the two images above, the top one may be just superficial damage to the gelcoat, caused by dropping a winch handle or something like that. The bottom one is a more serious impact and will probably require repairing the underlying laminate.
Grinding away the damaged gelcoat and any damaged layers of laminate and tapering the area around the damage to a 12:1 and then re-glassing the area—preferably from both sides with epoxy resin, is the way to go about repairing something like this.