The Gap-8 was the original name of Jack Reed's Gap-Air-Mirror. It features an eight man defensive line, a single player in a middle linebacker/free safety position, and two corners.
Most youth coaches with little knowledge of the game will dismiss this defense as unrealistic. Even a few highly experienced coaches will make the mistake of laughing at this simple formation. Be warned, this is a mistake, and it's a large one. This defense works like a dream at the youth level. Even in high school I've dropped into it on occasion to give opposing offensive coordinators fits.
The defense features six down linemen positioned in the gaps. Two additional players are outside linebackers that are head up to inside shade on the tight ends. The configuration is something like the 6-2, but trust me, the defenses are nothing alike.
Pass defense is comprised of three parts: a strong, six man pass rush, bump and run coverage from the corners and the outside linebackers, and one free safety/middle linebacker in cover one zone.
Typically, the free safety/middle linebacker is the defensive captain, however my personal choice is to make the Sam linebacker responsible for the captain's duties.
Here is a brief description of the Gap-8's defensive lineup and responsibilities. The diagrams should give you an idea on how the defense changes to "mirror" the offensive formation.
Figure 1 shows the basic alignment and penetration paths. At the lower levels of youth football the ends box to contain the edge, but at approximately the 11-14 year old level I would begin teaching the ends to contain using the upfield shoulder and hip to attack the ball carrier's upfield side. Using this "head across the bow" method of contain negates a speed advantage while still allowing the defensive end to use his inside shoulder to meet and deflect attempted kickout blocks.
Figure 1: Basic Gap-8 alignment and penetration paths.
Notice from the penetration diagram that only the Power and Strong tackles (Defensive Guards) are allowed to penetrate deep and "search and destroy". Both Defensive tackles are only allowed to to penetrate one step into the offensive backfield to guard against the off-tackle play. More importantly, both defensive ends are required to run the "L" shaped route into the offensive backfield as deep as the ball.
This technique of sweep stopping is called "Boxing". It has its usefulness in lower levels of football, but it is vulnerable to kickout blocks. Use other sweep defense techniques against higher level opponents.
Also note the paths that Sam and Willie take to the backfield. Their number one priority is to plug the tight ends and prevent them from going out for passes. This defense uses bump and run coverage to prevent the quick, inside release. By reading the tight end's movement, they can discern the type of play (pass or run) and the point of attack (if the tight end tries to put his head on the inside, the play is going inside. If he fights to put his head on the outside, the play is going outside). This takes a number of reps, but is both possible and relatively easy to teach.
Be warned! The description of the Gap-8 on this web site is meant to provide a rough idea of the defense for you. It will help you make an educated, informed decision as to whether or not this defensive system will work with your players and in your league. It will not show you how to attack every offensive formation. An attempt to do that would result in this web page taking eons to download. Figure 2 contains some basic alignment diagrams for the defense.
Figure 2: Basic Gap-8 alignments.
Further information on the Gap-8 is available at Coach Jack Reed's web site. I strongly encourage you to purchase his book, The Gap-Air-Mirror Defense for Youth Football, if you are at all interested in the defense. You will find that Coach Reed's defense features slightly different alignments than mine. Wherever there is confusion, I suggest using Coach Reed's material over my own, since my modifications were made to take advantage of things I saw in my own football league, and the personnel I had available.
Another source of material on the Gap-8 is my PowerPoint™ presentation on the defense, which I created to help my assistant coaches and players better understand the system. I have recently made the presentation available for download here. you will need to be running Microsoft Office™ or PowerPoint™ versions 1997 or later. You will also need a decompression utility called Winzip that can be obtained here.
I hope this helps. Good luck!