I have had two Gath helmets. the original and the newer style. I didnt like either at all. They have a very thin lining and the shape of the helmet means that I have high pressure points on my head that get very uncomfortable after wearing for a short while, and there was very little or no scope for altering them to fit comfortably. .
On top of that, they were really only designed for surfing where they are (as I understand it) more designed to prevent cuts and scratches, and less for major impact protection. Now this is not really much of a problem for windsurfing either, as serious hard impacts to the head are very rare and unlikely in my long experience (unlike my experience skiing and snowboarding!), but worth noting. Helmets designed for skateboarding, canoeing, skiing and snowboarding have always proved to be a far better option for me. They come in Multiple sizes and shapes and have thicker linings. Many have no ear covering, or optional cloth/padded ear covers (which I prefer). They are almost always slightly bulkier than than the Gath helmets as they have thicker padding/impact lining, but the only downside of that I have noticed, is to your vanity.

True, the thin, low profile Gath style are a bit better for duck diving under surf waves, but that is very rare when windsurfing compared with surfing. Most ski/snowboard style helmets also have far better warmth insulation for your head, which is surprisingly important if you sail in cold conditions with a high wind chill factor.

And the thicker lining of the ski/snowboard/skateboard type also often facilitates creating a convenient cavity for a small gps.
www.seabreeze.com.au/forums/Windsurfing/Gps/Wearing-the-Motions?page=1#6