In this book, the narrator acts as the readers' eyes and ears. We see Gilead as she sees it; we interpret it as she interprets it; and our only knowledge of it comes from the tidbits she gives to us. From a dramatic or plot standpoint, we only discover the narrator's history and the events that led up to the foundation of the Republic of Gilead as she reveals them, almost as an aside to her narrative about what's happening to her at her third posting, at this final Commander's home.
We have to trust her about Gilead and what happens to her. At the same time, that trust is continually undermined by her comments about how she wishes she could change the direction of her story and admissions about how she has changed it, as well as constant evasions and the use of pseudonyms. She even says at one point, "This isn't a story I'm telling," before turning around in an about-face and saying, "It's also a story I'm telling, in my head, as I go along" (7.34-35).
This both is a story and isn't, is both real and isn't. It's both truth (presented by Atwood through the lens of fiction) and reconstruction. All this hesitation is made possible by the narrator's simultaneous presence and absence, as she tells about what's happening to her and what has happened, while also revealing as little about herself as possible.