One thing the gapping looks for is the place(s) in your set where you have a "fall-off" in expected yardage for a club. Let's say a golfer finds this average yardage for irons:
- 7i = 140 yds.
- 6i = 150
- 5i = 160
- 4i = 165
- 3i = 160
In this case, the golfer "hits the wall" for anything longer than a 5i. The golfer could take lessons to try to improve the swing, OR possibly softstep the 4i and 3i to pick up yardage, OR, swap out the 4i and 3i for hybrids.
Gapping also looks for the lowest-lofted FW a person can hit well. Many average golfers find they can hit a 4W as far - or farther - than a 3W.
So, you get a mapping of your average distances, and a diagnosis of places in your bag where you may need equipment changes. Some of the recommendations may be for a different shaft than you're using right now.