When you sign up to a gym that has not yet opened, your initial payment secures the introductory offer available for that gym.
Monthly membership payments will not begin until one month after the gym officially opens. This means you will not be charged monthly while the gym remains closed.
Your membership will become active once the gym opens, at which point your introductory offer will apply.
Heads up, for Ultimate members, multi-gym access to newly added gyms will be available once the gym officially opens.