I would pronounce it GEE-neth.
Gyneth RosaGyneth NiaGyneth WillaGyneth LaneGyneth Tiffany
Gyneth is fresh, youthful and vintage.
Gwyneth is okay, but I prefer
Gyneth. Two related names I like are
Gwendoline and
Gwen. If I ever use
Gwendoline, I would use
Gwen as a nickname.