The Gehmann Zero Magnification Dioptre has been passed for use by the ISSF. However, although these are used to correct vision problems, they do not constitute a single corrective lens under NSRA rules and therefore cannot be used in conjunction with an Eagle Eye.
NSRA Rules say:
8.4.3 A lens may be used attached to or forming part of the foresight………………………… The lens may not be used in addition to any other lens or system of lenses except a single prescribed sight-correction lens. The sight-correction lens may be either attached to the rearsight or worn by the shooter.
126.96.36.199 Aperture. A rearsight, including tube sights, which may, if so desired, have attached a single lens or system of lenses not containing an aiming point as a substitute for, or in addition to, prescribed spectacles.
The Gehmann Zero Magnification Dioptre isn’t a single, prescribed sight correction lens. It’s an adjustable, off the shelf, sight correction, multiple lens system and so would not be allowed in conjunction with an Eagle Eye.
It can, however, be used with a foresight that doesn’t contain and Eagle Eye.