A key factor in picking out a gaff is the size of the hook. Gaff hook size is typically measured by the hook gap width; with 2,3,4, and 6 inch gap hooks being some of the most popular sizes.
Smaller gaff hooks (2 and 3") are great for smaller fish and "picking" fish off very easily. Medium hook gaffs (4") are great all purpose hooks for a wide variety of species. Large gaff hooks (over 4") are great for bigger fish.
It may not be practical to keep multiple gaffs on the boat, but if possible keep a small "pick" gaff and a larger gap hook gaff on the boat. The smaller gaff makes sticking small to medium fish easier, and the big hook is great when a true trophy pops up next to the boat.