Chromebuck, gap shooting is an aiming method using the point of the arrow. First, you find your "point-on" distance, which is the distance at which you can place the point of the arrow on the spot you want to hit and the arrow will hit that spot. For me, it is about 40 yards. Then you shoot at other distances, and find where you have to place the point of the arrow to hit your target. For example, I have to place the arrow point about a yard above the target at 50 yards in order to hit the target. If you want to use gap at distances under your point-on (I don't; I use instinctive for closer distances) you have to find out how far under the spot you have to place your arrow point in order to hit it.
It is critically important that you maintain your focus on the target at all times; the arrow point will be blurred in your peripheral vision.
Your gaps may change as you change arrows or arrow length, so may have to be recalibrated some.
There are two kinds of gap shooters: one actually measures the gap (1 yard above the spot at 50 yards); the other just notices the gap and doesn't associate a particular gap with a particular yardage. Over time, they are able to set their gap without knowing the actual yardage or gap (could this be called "instinctive gap?" I dunno). Howard Hill shot this way.
Some very good shooters, like Rick Welch, state that they are instinctive shooters, but also state that it is very important to be aware of your sight picture in your peripheral vision. The sight picture includes the arrow point, the sight window, and whatever else you might see in your peripheral vision, such as part of your bow hand. To me, this awareness of the sight picture makes this form of aiming very similar to Howard Hill's method, and Howard said he was not an instinctive shooter. So sometimes the distinctions can get blurred.