We slept in what had once been the gymnasium.
Plural pronouns immediately show loss of individuality, part of the collective.
Atmosphere of change and fracturing of time.
Air of normality, the setting more familiar for the reader and makes it easier for them to imagine these events happening in the world that they know. Links to Atwood's aim to highlight the cyclical nature of history and to warn the readers of making the same mistakes of the past.