Does anyone know if a geofencing push notification can be delivered to a customer from SFMC even if they have force-closed the app?
If implemented correctly, yes, the SDK will show geofence (and any other Marketing Cloud related messaging) whether the SDK is running or not.
I say, "If implemented correctly" because the SDK must be initialized in the Application's Application Class'
onCreate() method. When the geofence is broken Google Play Services will broadcast an intent that is picked up by the SDK's receiver. If the application is not running, the receipt of this broadcast will start it resulting in the SDK being initialized and the notification message being shown.