Boat has to be moving fwd. If you are lucky they come right along side. Long gaff with a small hook. If you miss once God help you.
It helps to have already boated a few so you are calm. When it goes right it is zen-like...
We have head gaffed three big ones and bounced them off the gunnel overboard just this season.
Once its in the boat you are only halfway there. I have lost a 20# dolphin that was gaffed, in the boat, fell off the gaff, flopped and sprayed the entire boat with blood, threw the hook, bitch slapped me as I tried to grab it and then went back overboard.
If your boat is not in gear its damn near impossible.