I have the older gath convertable. I liked it when I first got it, mainly for the spf protection. As the visor aged, I could no longer ride full faced due to scratches on the lense, and would ride with it like a visor. For reference, I have used the Protec Ace Wake. Sizing is comparable. For me, if buying a helmet too small, the gath was snug around the ears, whereas the Protec is snug around the cranium.
With respect to head protection, the Gath works fine for things running into my head (boards, etc). But I wouldn't expect a lot of brain protection from high speed wipeouts, and would prefer the Protec due to the extra foam. The Gath is better than no protection at all. No helmet will protect from every impact, the idea is to try to minimize the damage.
Historically I used the Protec in the surf and the Gath for flat water riding. I am now just using the Protec and am using UV glasses.
For hydrofoil riding, I would prefer a helmet with full face protection.
I had a Gath convertible. It was the least worst helmet I have owned. When it worked it was fine. Problem was the size adjustment comes from stick on strips. The glue was hopeless and the strips would come off. If you have a crash and the helmet comes lose then it almost tears your head off. Massive bucket effect. I could have stuck the strips on using a permanent glue, but I would have had to buy the glue and do the sticking and and get it right etc. I was not interested in doing that on a $200 plastic hat.
I am not interested in any of the currently available water helmets. The bucket effect and wind noise is too bad for them to be usable. I know, i have owned most of them. A helmet has to be about as comfortable to wear as a hat for it to be useful. The best helmet is the one you want to wear. All the existing ones are not a pleasure to wear, and I don't believe they are designed for what we experience kiteboarding and foiling.