Gelcoat is the first layer laid in the mould when a fibreglass boat is built, so it ends up on the outside. It's a pigmented resin, thin and hard, and it's what you see and polish. Behind it is the laminate of glass and resin that carries the loads.
Being thin and hard, it cracks when the laminate behind it bends or takes a knock. The pattern of the cracks is the clue to the cause.
What a surveyor checks
The whole hull and deck in low light, crack by crack. Safe Skipper reads star cracks as impact, with the centre at the point of the blow, and parallel or linear cracks as bending: along deck edges, or running out from a fitting that loads a panel. Then the laminate round each crack, pressed and sounded. Firm and solid is reassuring. Soft or moving points to delamination or a wet core.
Below the waterline, gelcoat is where osmosis blisters form.
In the report
"Foredeck, linear gelcoat cracks radiating from the port bow cleat base. Laminate firm on sounding. Check the backing plate and rebed." See gelcoat stress cracks.