Hello

Please elaborate on what's the External/gateway IP address is. Is that a value of the field that you collected via some input module? Do the original log messages contain the info about the gateway IP address? OR Is it a value that you need to collect from the OS (via bash or PowerShell) and insert it into the custom field called $GatewayIP? In your example, you set the $hostname field value to constant - 'test_SRV'. Custom fields can be added by using the NXLog language and an Exec directive, and if your Gateway IP is a constant, then just add an Exec block:

Exec $GatewayIP = 'XYZ.XYZ.XYZ.XYZ';