The Yabby reports it's battery status ('good' or 'bad') in the Status Flags in the Digital data field. It is active (1) when the battery level is good, and inactive (0) when it is low.
In Telematics Guru, this corresponds to Digital Input 25 - see the bottom of the image.
Yabby default I/O mappings in TG.
To set up a low battery alert for the Yabby in TG, use the New Alert Wizard and select the Low Battery Alert.
Go to Admin -> Alerts -> New Alert Wizard
Select the Low Battery alert, and apply it to your Yabby (and any other battery powered assets - it works for them all).
Select any recepients for the alert.
Save the alert.