If you are within Bluetooth range and have Bluetooth enabled on your mobile device and in the SensorPush app (learn more in this article) you can still receive alerts and data via a local Bluetooth connection, just as you can with no gateway at all.
However, if the internet or power goes down, causing your gateway to go offline, while the system cannot continue to monitor the sensors, our cloud checks the status of each gateway approximately once per hour. If we find that a gateway has gone offline for any reason, the cloud will send a notification to your mobile devices (provided they have internet connectivity, notifications enabled, are not muted, do not disturb is disabled, etc) that informs you of the missing gateway, so you can be aware of the possible issue and that monitoring is temporarily offline.
Additionally, in the main device list in the app, each gateway on your account is listed, along with its connection status. The gateway updates its status with the cloud approximately once every 5 minutes. You can force the app to check the status in the cloud by scrolling to the top of the list and pulling downward to manually trigger a data refresh.