I had a Gath helmet for surfing until it was stolen.
They are way too lightweight for kayaking, in my opinion. Also, they are WAY overpriced. They have a small niche market and can charge what they want.
A surfer is not going to wear a bulky helmet that drips water in their face so those things are featherweight, fit very close to the skull, and drain water away from your eyes. The tradeoff for these features is that it would not take much to knock you out with a Gath on. Surfers rarely hit their heads and the vast majority do not wear helmets. The purpose of a Gath is to prevent your board from knocking you out if you fall on it or to increase your chances of survival if you piton your head on the reef. They are not designed for going upside down in a kayak.
MAYBE I would wear one for playboating, but only if I couldn't find my kayak helmet. I wouldn't wear it for riverrunning, even in class II or III.