This names a SYNC gateway that the device tracker can use. The gateway must be enabled. A device tracker gateway can only use one SYNC gateway.
The SYNC gateway is a TCP/IP-based gateway that supports notification through a persistent connection. It is the recommended gateway.
There is a predefined instance of the SYNC gateway called Default-SYNC. It is configured to work with the predefined device tracker gateway, called Default-DeviceTracker. When the Default-SYNC gateway is used through the Default-DeviceTracker gateway, delivery confirmation is turned on by default.