“Kiriona” is the Māori form of “Gideon”.
In The Locked Tomb, Gideon going by Kiriona (in addition, mind you, to Gideon, since she still remembers and uses that name, as well) is an instance of a Māori woman connecting with an aspect of her Māori heritage she previously wasn’t aware of due to having grown up an orphan.
Gideon getting the name Kiriona isn’t her losing anything; it’s her gaining something. Thematically, Kiriona in the third book has gotten or gets all the things she wanted in the first book (a Cohort officer commission, legitimacy, Crux dead etc), both so that she may know if she truly wants them or not and so that when she gets Harrow in her life, her whole life isn’t just revolving solely around Harrow.
Anyway, I’d encourage members of the fandom to think about how they engage with the name “Kiriona” and how they frame Gideon using it in fandom discussions. Treating Gideon using the name Kiriona as something bad or wrong or lessening risks coming off as really insensitive.