This is a companion novel to The Giver-- not a sequel. It does not pick up Jonas' story, he's not in it. Just so we're clear.
Gathering Blue gives us another view of what the (post-apocalyptic) future could be. (Hopefully not though, yikes.) It's a society existing separately from and yet at the same time as Jonas' society. (There's even a blue-eyed boy mentioned at the end. Is it Jonas? Could be.)
Kira's alone in the world. Her father died before she was born, her mother has just lost a battle with illness, and the neighbor women want her land to build (I kid you not) a pen for the "tykes and the fowls" so they won't have to chase them. (Now, I'll be the first to admit that there are perks to something like that. I block Mister into the living room with boxes. But a pen surrounded by thorn bushes? Seriously? So they can play in the ashes and debris that used to be Kira's cott? With the chickens? Yikes.)
Kira herself is saved by a Guardian who defends her and brings her to the Edifice to live and work with the threads, as her talent has out-classed even her mother's.
But things may not be what they seem. And just because the door is not locked does not mean she's free.