Assuming it does fade, that just means that the phenomenon Gray underwent was more advanced. Not particularly surprising, Gray is a natural Sibyl and Shirou isn't, and the reaction that she underwent physically altered her entire body and facial structure, while Shirou just got some memories and skills. Though I feel like there's an element I'm missing. Shriou was reacting to himself in fairly close proximity, and only got mental and spiritual changes that may have proven temporary. Gray was in proximity with a Noble Phantasm of a distant realtive of hers whom she was reacting to, with said distant relative being on another continent. Even if she's a natural Sibyl, I feel like I'm missing something that made the changes so deep and permeant. Maybe the village's cult created a Foundation that enhanced it, but my gut says I'm missing something. Logically, I think I'm drastically underestimating how much easier spiritual invocation is for a Sibyl. That might be it. Lord El Melloi II did say that it's sometimes called Sibyl's magecraft because of their affinity for it. And the way she could percieve the cat's residual thoughts when Lord El-Melloi II had no idea, even though he's a trained (but ordinary) magus and Gray's skills aren't... exactly nonexistent, but so simplistic and basic she's not even recognized as a mage at all. Hmm. Yes, the more I think about it the more I come to believe that I am underestimating the importance of innate traits to magecraft, much like Waver did.