it's the first time I've heard that pressure wash causes gelcoat damage! Was it a particularly strong pressure wash? I wash the deck with pressure wash a couple of times a year, always before polishing, I haven't noticed anything unusual.
I think it's more likely to be a case of micro bubbles in the gelcoat, which are then broken by pressure washing and form voids in which dirt and mould can loiter. I had the same problem on some areas of the superstructure on a Hallberg-Rassy I owned.