Get a harpoon. I used a 3 ft. "tuna" gaff too long- too many ugly battles with 80 - 100 Lbs. Quickest way to scar your boat is with the point of the gaff hook when you are trying to fight those big ones. Starting last year I got a harpoon with 2 heads from a friend. Much better for any size. Hit them behind the head and the point goes through like butter. They bleed out nicely. Two or more heads allows you to keep fishing while you have fish out on the tether.
I agree about not allowing the head to come above water- they are docile when below the surface- can even rub their backs, but let their heads come out of water, or miss a gaff stab, and they go crazy.
Get them in the fish box with a tight lid cause sometimes a "dead" butt seems to come back to life- might just be nerve reflex but big ones can really thump on the boat.