If you are far-sighted and can't see the tip of your arrow in your peripheral vision at anchor, than you may have a serious problem that would prevent you from gap shooting all together. You got to be able to see the tip of that arrow in your peripheral vision at anchor or how else are you going to set your gap.
Try this! Take just an arrow, no bow, and get in position like you where going to draw your bow. Hold the nock at your usual anchor like you would at full draw, and focus your eyes on the target. Now you’re in a relaxed position and can practice looking at the tip of your arrow in your peripheral vision at anchor, and are not under any strain. Try this at different distances, and adjust your gap in relation to your distance. It's going to be a little blurry at first, but the trick is to get used to the blurryness where you can effectively use the point of impact (aka. the target) that's in focus, and the tip of the arrow that's out of focus and use the two to set a recognizable gap that can be measured so it can be repeated.
After holding the arrow at your anchor for say 15 or 20 minutes or how ever long it might take, If you can't train your eyes to see the tip of your arrow in your peripheral vision at anchor, than I doubt you will be able to use the gap method very effectively, and probably won’t be able to gap off the riser either.
Just my opinion, I could be wrong.