To monitor the Gateway to check it is up.
Navigate to System → Routing → Gateways → WAN_PPPOE:
Click Edit.
Monitor IP: 9.9.9.9.
NOTE: This allows pfSense to make sure the WAN_PPPOE connection is up.
Not all Gateways respond to ICMP request so this gets around that issue.
This is needed to control what happens when your connection goes down.
You can use any reliable IP you want.
Make sure you check the IP responds to a PING manually so you know it will work for the monitor.
pfSense creates a static route to 9.9.9.9, which is hidden.