Relax gateway checking
During network lessons at school you learn that the gateway must be reachable with an IP in the same subnet has the host.
Few people understand that this gateway IP is is just used so that ARP get the mac address of the gateway.
So, there is nothing wrong using a gateway IP outside of the interface subnet.
This kind of unusual config is used by OVH, a french datacenter provider (and probably other) with a system they call 'failover IP'
The base machine have a normal config ip/netmask/gateway . Failover IP have /32 netmask and no gateway.
When the base machine is a ESXi vmware server, it receives the normal config. The virtual machines receive the 'failover IP'.
Some config tricks allow the linux/windows/bsd VM to accept the gateway IP provided to the base machine that is completly different from failover IP.
pfsense do not allow using a gateway in this case.
Say you have 126.96.36.199/32 failover IP for WAN1 and 188.8.131.52 for gateway.
Let me enter this config or let me give pfsense the mac address of the gateway.