I'm not sure if this will work. You want the alert to be triggered based on: "Certificate will expire shortly for Secure Gateway at address".
But that is showing somewhere in Horizon. In order for the alert to be triggered, that same line has to be reported to vROps. And I'm not sure if that is the case or not.
If you are using Skyline, It could be that the Skyline MP already has an alert for cert expiring. Did you check for that?