If your gym uses access control, you get in with the app. Under the middle button, at All, is Open door.
The code. You get a code that you hold up to the reader by the door. It works once and only for a few seconds; the app counts down. Expired? Tap New code.
Why so short. A code that only works briefly can’t be photographed from your screen by anyone and used later.
Can’t get in? Then your subscription may have been paused, for example because of payment arrears, or your subscription may only give access at certain times. Look under Profile → Your subscription, or ask at the front desk.
No button showing? Then your gym doesn’t use access via the app.