My only guess would be that she knew it was going to happen, just not how, when, or by whom.

Anyone else besides me think that Gladys volunteered to sacrifice herself, and that the Guilty Remnant killed her? It seemed like the "head-nod" in the opening scene, plus Laurie's flashback to the final stone being thrown (by her presumably), infer that it was an inside-job. Someone on another message board hinted…