Gwendoline was not in Army of the Dead because, despite the final scene of Army of Thieves taking place "sometime later," insufficient time had actually passed for her to be released from prison or to escape before the events of Army of the Dead unfolded.
Understanding the Timeline Between Army of Thieves and Army of the Dead
In Army of Thieves, the prequel to Army of the Dead, Gwendoline sacrifices herself to allow Sebastian Schajen-Bartz (later known as Dieter) to escape. She is last seen being taken into custody, but not before telling Dieter that she will find him again.
Although Army of Thieves implies a significant passage of time with its "sometime later" title card at the end, leading viewers to believe Gwendoline might have had the opportunity to rejoin Dieter, this was not the case. The narrative timeline establishes that Gwendoline was still incarcerated when the zombie apocalypse, depicted in Army of the Dead, began and progressed. Therefore, she was unable to participate in the vault heist or even be present in the world outside prison during the main events of Army of the Dead.
