A great place to learn Gap is on NFAA\\IFAA Field rounds,they tell you the distance and the size of the target so it's easy to work out a reliable and accurate Gap system, once learnt it's pretty easy to transfer onto unmarked 3D\\Bowhunting shooting.
If you read Tony's book he goes into detail about subtle differences in all the aiming methods, I tend to use Gap and Point of Aim on Field rounds and Instinct and Split-Vision more on unmarked 3D as they're more about feel and experience and less dependent on knowing exact yardage. If you get a 3D tourney with tough terrain it can be very difficult to estimate exact yardages and know the size of the 3D.
Gap does work on unmarked 3D but it's better suited to marked yardage shoots. I suggest like myself learn all the different aiming methods and just pick what works best for that particular shot scenario.
A couple of ways to learn is put a bright dot on the target roughly where you're Gap is, keep adjust the dot till your arrow starts hitting where you want, the other method is to Gap off Riser put stips of tape on riser for each distance (like a primitive sight) till you learn the sight picture for each distance, once you take off the tape you should still be able to viualize the riser sight picture. Just stick to one distance till you learn that Gap and move onto the next one back, you will see a regular pattern as you move farther back as the Gaps are uniform. When I Gap I'm using both the tip of the arrow and Riser sight picture.
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IFAA World\\European Field Champ, Fita 3D World Champ