Firstly, the best advice I've seen on how to gaff a hali was written some time ago by Adi:
http://www.bigwatersedge.com/bwevb/s...alibut+gaffing
However, for me personally, on the kayak I like to float them by horizontally (rather than letting them "hang with their head above their tail") and hit them with an upward, vertical stroke, hopefully hitting them in the brain or, on the other side, in the stomach.
With that in mind, I've redesigned my gaff incorporating two distinct features: and ergonomic handle to keep my wrist from getting tweaked, and a somewhat different shape of the hook itself. For that vertical upward shot with the gaff I think the piercing part of the hook penetrates better being completely straight, and the sharply angled bend seems to keep a thrashing hali from easily sliding off. Also, it's a bit wider in gap than a standard hook so, theoritically, I can get nearer their center line.
Time will tell...