We rarely use the gaff. (other than for taking fish out of the fish barrel) If we're worried about a fish getting away, we'll try to get another arrow into it and use the two arrows to lift it.
We do use the gaff for bigheads. They're soft enough that they're easy to gaff. Gar are hard to gaff. Big commons can be tough to gaff too.
If we should be lucky enough to get into those big fish down at the Muzzy Classic, we'll keep the gaff handy. Might be a bit tough lifting a 70lb grassy without it. [

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It's like anything else. If you don't have a gaff you'll probably need one and kick yourself for not having it. If you buy one right away, it will probably months before you really need it.