Gelcoat crazing is a problem with some boats because the resins they used in building them had some sort of chemical makeup that led to the crazing ---- years after they had been built. It seems to have affected multiple builders and taken a long time to figure out. They changed the mix and the problem disappeared - except for the boats that had it. It seems to only affect the gelcoat - not the underlying fiberglass, which is applied in a separate step, after the gelcoat, in the mold. Cracks in the gelcoat CAN indicate problems with the underlying fiberglass being too weak, flexing, and leaks. This does not seem to be the issue here, but a surveyor would help to confirm this. If the gelcoat alone is crazed, it is a cosmetic problem. It can be fixed, or ignored. To fix it calls for stripping off all the gelcoat on the deck and re-applying it. We saw a J/42 that had had this done. Looked almost new, and the asking price reflected this. We also saw one that had extensive deck crazing, and no leaks, asking for less than half the price of the first. Depending upon the size of the boat, $20k might be low for a re-do, but it's likely the right ballpark. 2-part poly paint (Awlgrip etc.) might offer a temporary fix, but would likely be in the same $20k price range unless you did it yourself. And it would only be temporary. The cracks will end up becoming visible again through paint. Some people slap on some wax or other compound to fill the cracks and go sailing instead of worrying about it. The broker was right to tell you. Now you get to decide how important it is to you.