And why does Gideon think killing the Black Fairy will give him Savior powers?
I’m a tiny bit frustrated by this.
In what part of the OUAT universe does Saviorhood work like the Dark One’s Curse? It never has; Emma is a Savior because she’s the product of the truest love and because of Rumple’s machinations with the Dark Curse. Aladdin is a Savior for some vague handwave-y reason involving being a thief with a heart of gold and a “diamond in the rough” (whatever that means).
Emma’s Saviorhood and Aladdin’s Saviorhood has little in common but it does have that neither of them had to kill another Savior in order to gain their status as Savior. Gideon is motivated by his own horror of a childhood and some sort of innate goodness that fuels his desire to save others from the Black Fairy–which in and of itself is interesting and a good narrative jumping off point–but confusing the Savior mythology (even more than it already is after the introduction of Aladdin) calls Gideon’s character into question. You could theorize that Gideon’s belief that killing the Savior would make him a Savior came from the Black Fairy but if Gideon has shrugged off all of the fairy’s other lessons (like hatred and evil) then why would he follow this one?