Gwendolyn does not appear in Army of the Dead because she remains incarcerated at the time the film's events take place.
At the conclusion of the prequel film, Army of Thieves, Gwendolyn (Gwen) is shown surrendering to the authorities. This act allows Sebastian Schlencht-Wöhnert, who later becomes known as Dieter, to escape. Although the final scene of Army of Thieves indicates that "sometime later" has passed, the period is not long enough for Gwen to have been released from prison or to have successfully escaped before the zombie outbreak and subsequent events of Army of the Dead unfold. Her continued confinement is the direct reason for her absence from the later film.
