I'm also having problems with Gateway monitoring via ping, but I'm not sure that it is the same problem as yours.
You would not necessarily be able to ping that gateway from your LAN, depending on which WAN is used to do the ping.
Since you have multi-WAN, a ping might go LAN -> WAN1, or it might go LAN -> WAN2.
When you do your ping test from the pfsense box, you need to specify that the ping go out the same WAN that is using the gateway you are trying to ping.