As I understand the problem, it is not so much the gelcoat as the difference in thermal expansion/contraction rates between the surface coat (Gel or PU) and the underlying composite structure (Fiberglass/Carbon). As the temperature changes rapidly (fast ascent or descent through a significant temperature gradient), the two materials expand or contract at different rates. Too rapid a temperature change can cause cracks to form in the thin and fragile topcoat.

Bob LaCovara (sp?) gave a very good lecture at (I believe) the 2006 SSA Convention in Ontario, CA that addressed this and other interesting facts about composite structures and finishes. One fact he pointed out was that there are many different types of "gelcoat," and the sailplane industry has been a very tiny segment of the market, utilizing gelcoat formulations that are nowhere near the technical superiority of more modern products, and what the industry does use is often applied incorrectly and in a much thinner layer than what was intended by the manufacturer.

Indeed, the different types of gelcoats used by various sailplane manufacturers often exhibit widely disparate results in longevity and appearance. Some of this even manifests itself between examples of the same make and model glider depending on how the gelcoat was originally applied in the mold. In the 80's, it was assumed that there wasn't any difference between laying the first glass fiber and epoxy resin into place immediately, or a day after the gelcoat was sprayed in the mold. Only after fifteen or so years would it become apparent that crazing and cracking was more common in the gliders that had a delay of more than a few hours between spraying the coating and sealing it from the air with the fiberglass layers.