See About alerts.
Symantec Web Gateway supports SNMP version v2 and version v3. Symantec provides Symantec Web Gateway MIB files to import into your Network Management System. These MIB files define what monitoring information Symantec Web Gateway provides the Network Management System. The following Symantec Web Gateway information is available by SNMP:
Consult your SNMP documentation for configuration information. Specify the SNMP information and download MIB files on the Administration > Configuration > SNMP page. You can configure Symantec Web Gateway to send malware alerts or system alerts as SNMP traps. Symantec Web Gateway sends alerts as SNMP traps in real time. Alerts indicate changes and minor to serious issues on Symantec Web Gateway.
Symantec Web Gateway does not support management by SNMP. You cannot use a Network Management System to make changes to a Symantec Web Gateway appliance. Instead, you can use a Central Intelligence Unit to make changes to one or more Symantec Web Gateway appliances.