Quick answer is gap is a method of shooting in which one looks and uses the arrow as a reference while knowing how far to hold the arrow below a target. For example, if you have target at 20 yards, if you below your target say 20" it should hit the bull. It takes experimenting to setup your gap, but once done it's a very effecient shooting method on targets and hunting.
Instictive is shooting an arrow basically using muscle memory and repeated reptitions ground in. You don't reference the arrow at all, you stare or burn a hole in your intended target, put the bow on target, and release.
Both are accomplished through practice of form over and over, just like anything else that is learned or required.
Rod Jenkins explains gap very well in the first Masters of the Barebow DVD. Instictive is also shown, with a great demo of done by Dale Karch of 3 Rivers Archery (if I remember right he references off the arrow, but doesn't have a set gap, he refers to his style of instictive). The DVD is a great video to see different styles and ways of approaching things. Like it has been said, nothing works the same for everyone, but something can work for someone.
Not sure if I answered the way you wanted and I'm sure you'll get some other answers. Good luck in which ever you try, but do yourself a favor and try at least both. You never know what it takes until you at least give it a honest try.
There's also point of aim and split vision, which are along the line of gap shooing, different variations of it though.