I dont worry about cleaning my fish while otw. Less possible injury to oneself, plus if you decide to fillet, there are certain size limits of the fillet that must be adhered to. I echo what the others say about bleeding. You can also take a couple of frozen water bottles and shove one down the throat of that yellar to help keep it cool
One other thing that I do is cover my catch with a wet mesh sack (the kind you get for the cattle boats).
As far as gaffing, I have yet to gaff a yellow. I just grab the tail, and hang on tight! I wear gloves, so I am able to get a really good grip. That is just my preferred method. I did gaff my hali, and the only thing that I can think of as far as advice would be to gaff with the point of the gaff away from the yak. The reason for this is because after you stick it, the fish will most likely be pissed and you will probably use the side of your yak for leverage for the gaff. If the point is against your yak, you might risk a puncture! I knew there was a reason why I carry a small roll of duct tape with me!
Mark