A Great and Terrible Beauty is written in the first person present, which means we experience everything from Gemma's perspective, right as it happens. For instance, while Gemma walks with her mother around a bizarre in India, she narrates:
The flies—my most ardent admirers—dart about my face. I swat at one of the little winged beasts, but it escapes and I can almost swear I hear it mocking me. (1.6)
We are right there with Gemma, so close we are almost bothered by the flies too, and as the book progresses, we never leave her side.
Gemma's a dutiful narrator, and she tells us everything she does, sees, hears, and feels. It's almost like—for the duration of the book—we are Gemma, and since she's at the center of the story, this keeps us close to the action and invested in how things turn out for her.