""The Gospel of Thomas declares that a woman cannot be saved unless God first changes her into a man (the very last verse of Thomas, 114)."
When a woman (or a man, for mankind is the feminine aspect or spirit) is saved, that is when the transformation starts and the feminine prepares to becomes one with the masculine. That's what the marriage is. A twaining of your mind and the mind of Christ and we become bone of his bone and flesh of his flesh, in a spiritual sense. Your will is sublimated to the will of God, as Christ sublimated his will to the will of the Father when he said, 'Not my will, but thine be done.'
Now, in that He is now the eternal mediator between man and God, it is for us as individuals to sublimated our will to the will of Christ, (our husband, if you will) who is the express image of the Father within us, for Christ dwells within us and we in Him. Ideally, in conscious internal dialogue.
So I see no problem with the Thomas statement, though I wouldn't take it to mean a natural happening, for what profit has the flesh? The statement could serve as a metaphor for the wedding between Christ and His church, a confirmation of the inner process of re-uniting with God and realizing our salvation on an individual basis.
Sorry, but you're simply reading into it what isn't there. This is not anything like St. Paul ("There shall be neither... woman nor man"); there is no reciprocality indicated. Nor did the gnostics believe there should be. Besides, the bride does NOT prepare for the groom by becoming groom-like; in fact, preparing for the groom is done by bringing forth the fullness of femininity; so reading it as a preparation for the wedding feast is precisely backwards.
Further, the gnostics wholeheartedly, passionately, and fundamentally rejected the wedding element of the Christian faith. The early Christians saw marriage as a foretaste of the ecstatic union, Mary and Joseph experiencing the presence of Christ so closely that they had no need for the shadow of it that is sexuality; the Gnostics went in the opposite direction: to see sexuality as a curse created by the conquering of dirt over spirit, and religious sexuality as an abomination, seeing the "sacrament of marriage" as being of the same cloth as temple prostitutes.
So, yes, they did see "unsexing" as a preparation for their union with God, but that was because their god hated marriage and sexuality. Our God created sexuality as a way of permitting us to experience the bond between love and creation.
One last point, more of a pop-culture reference: The cosmology / "religion" professed by the vampries in Buffy the Vampire Slayer is actually quite similar to gnosticism, minus the ascetic elements. Why? The gnostic view is that the experience of a spiritual longing was an evil created by the Hebrew God, a lesser God who revolted against the Gnostic God, according to them. Vampires were, then, humans without the Yahvistic spark of spiritual longing, experiencing only "pure" animal longings. (The Gnostics did, hwoever, believe that there was no chance at going to the pre-edenic "natural" state, and sought, therefore to go to purity in the opposite direction: pure spirit.) Also, The Gnostic cosmology created a perfect viewpoint of Satan's arrogance: that it was God who messed everything else, and that he was the true god.