Gateway up/down email alerts

[user1234]

I am trying to get alerts for gateway status. e.g. I would like to receive an email saying gateway x went down at 12:34 03/11/19 and then another email when it came back again.
Unfortunately if the gateway goes down it can't send an email as their is no internet connection so the email would have to be cached until internet is restored. My old firewall used to do this perfectly but I can't find a way to do this on opnsense.
I have found the monit gateway_alert service but this doesn't seem to work for me, possibly because I am not using gateway groups or because the email notifications are failing to send due to no internet.

Is is possible to get opnsense to do what I want or am I better off using an external system to monitor the gateway?

[spetrillo]

I am also interested in doing this.