Gap shooting, in a nutshell, is using the point of your arrow like a sight pin. If you used to shoot a compound, it's like using one pin. You have to be lined up (proper form) to get the left and right, but other than that you just have to be consistent with your anchor, release, and follow through (as with any bow), and you have to be a pretty good judge of distance. Then you just hold the "pin" over or under the target, depending on the distance. The gap is how much you hold over or under at a given distance. At 20 yds, you may need to put the point of the arrow 6" below where you want to hit--that is your gap at 20 yds. The range where you put the point of the arrow exactly where you want it to hit is your point on distance. Lots of the better target archers gap. I only use it on long shots these days, but it helped me be consistent as I developed my form, and can be a big help as you learn to shoot "instinctive".

Chad