A gut hook is a small sharpened hook ground into the spine of a hunting blade near the tip. It is used to open the hide of game from the inside out without cutting into the muscle or organs underneath.
The point is inserted under the hide, then the hook is drawn along. The hide is trapped in the notch and cut from beneath, so the blade never has to press down toward the meat. It is a controlled zipper cut, which is precisely what field dressing wants.
Because the hook sits on the spine, it does not interfere with the main cutting edge at all. The knife works as a normal drop point the rest of the time.
A gut hook is a small, deep, curved edge, and no flat stone reaches inside it. It needs a small round file, a tapered ceramic rod or a purpose-made gut-hook sharpener.
A dull gut hook is worse than none, because it tears rather than cuts. If you buy one, buy the rod at the same time.