The Gath is a well made helmet with low drag characteristics. It doesn't have a lot of padding and that MAY limit its effectiveness in some of the more rare, higher speed impacts possible in kiteboarding.
Helmets protect your head in a few ways. First the shell, ideally, should stop piercing of the helmet and in turn your skull. Aside from slingshotting boards, most of the other impacts will likely be with blunt objects, i.e. the beach, pavement, walls, trees, etc. So the piercing potential should be less in many of these types of impacts. Particularly if you fix the main source of piercing impacts by ditching your board leash. There is a good chance that a slingshotting board could pierce many types of helmets.
The second and critical way in which helmets may protect you from impact is to slow down the impact by fractions of a second. The resulting lengthening of the decceleration time, even by hundreths of a second, can potentially reduce injury to your brain through impact against your skull and torquing. The deceleration of a given helmet is a function of the shell material but more critically of the type of absorptive foam and the thickness. Motorcyle and flight helmets have superior impact absorption qualities. Unfortunately, they are totally unsuited for kiteboarding because of many factors, weight, drag on impact with water, poor water drainage characteristics,etc. You need to find a balance between type of padding and thickness, shell resilence, light weight, low drag and protective value.
So, helmets should be worn to try to reduce the degree of injury and improve the odds of survival in a lofting or dragging accident. Of course they can do wonders to fend off the odd wave driven, low speed board impact too. There is NO ASSURANCE that any useable helmet for kiteboarding will spare you from serious injury or death in a severe impact however.
The main thing that will save you from that is to never go there in the first place. That is using proper training, good judgment and safe kiteboarding practices to AVOID the accident in the first place. It is like not driving off of cliffs because once you have there is not much that you can do. All the cure comes in the prevention.