Did Imu chose Gunko as vessel because of her potential or because he's a gooner?
Maybe Imu uses princesses as servants, and that's why he/she wants Vivi.
Gunko was maybe the princess of the kingdom where Brook worked as a guard?
Either way, Gunko seems to be an amnesiac (has memory loss) and one of her old memories was awakened just now.
But contrary to what some believe, it seems Gunko was fully in control of herself and not a mindless puppet until Imu possessed her in this final page, so it does not excuse her evil behavior even if she has memory loss.