It would be a terrifying experience. Pure titans lack intelligence and are driven by basic instincts like hunger. You'd be a mindless monster, perhaps constantly roaming in search of humans to devour without any understanding of what you're doing.
The character could start to develop some form of self - awareness over time. Maybe through repeated interactions with the environment or other titans, they begin to recognize patterns and develop a very basic sense of self.
Gabi remains alive in the manga. The plot doesn't lead to her death. It's one of those character arcs where she continues on despite facing various challenges.
For a more villainous touch, including Voldemort or his followers could create high - stakes situations. Bellatrix Lestrange could be a terrifying adversary for Harry and Gabi. On the other hand, adding new original characters, like a powerful but reclusive wizard who lives in the Forbidden Forest, could also make the story more exciting.