This is a "Wild Mass Guess" entry, where we pull out all the sanity stops on theorizing. The regular entry on this topic is elsewhere. Please see this programme note.
Gerd is really in the slayer line, and the entity she tirelessly pursues is guarding the local Hell Mouth, located below the glacier. She is reckoned a nut case and a cloud cuckoolander by almost everybody, because of the sunnydale syndrome shared by almost everyone in the area. Naturally, when she eventually gets the entity shot, the entire Hell Gate collapses, taking everyone with it.
Her interest in Brand is connected to the fact that he was meant to be her watcher...