Y'all are mixing some 'gator myths with facts. To start with, gators don't have a very big brain, so brain shots are not a good idea. They have a small brain referred to as a primitive reptilian brain. Don't let the description fool you, it just means they are very good at doing what they do, but they don't do a lot of thinking on ethics and math. If a walnut sized piece of brain fell into the gator's mouth, then that was his whole brain, so they guys missed the brain. They probably got muscle tissue that is all around the sides of the brain case, under the eyes, etc. that is used for the jaw muscles. The tissue was probably highly tenderized from being extricated via gunshot, so it might have looked like brains, or then again, the biologists might have known a lot about gators or crocks, but had no real idea what a brain looked like after being extricated via gunshot.
Gators do have dermal ossicles which are porous bony plates that are in the skin (note the bumps or ridges on a gator's back are from these). In the big animals, these plates can be fairly large and so can do some bullet deflecting. They do not make the animal bullet proof any more than your skulls makes your brain bullet proof.
If you get to use a knife on a gator, that means you are close enough to be bitten. That can be a really bad idea as gators can have reflexes quicker than yours. That being said, if a gator does have you, then a knife is a fine tool, but so is a gun.
I have watched gator hunters go after gators for the meat and hide market. When hooked, they don't use big bore rifles unless they have a tremendously big gator. The use a .22 lr and do shoot the head and try to hit the brain. The one advantage these guys have is that they have a stationary gator and a point blank shot, plus their lives are not being endangered when they do it.