I have two LANs (two NICs) and one WAN (IP=xxx.xxx.xxx.18). My ISP has provided me an extra public IP address (xxx.xxx.xxx.20). I can set up a virtual IP for .20 and port forward to the appropriate server. Is there a way to set up a virtual IP (.20) as a gateway? I want LAN 1 to use the normal WAN gateway (.18) and LAN 2 to use a virtual gateway (.20)? I want all LAN 2 traffic to look like it comes from the virtual IP (.20) and port forward that virtual IP (.20) to LAN 2 servers.
That isn't a gateway in the traditional sense.
What you want is manual outbound NAT. Firewall > NAT, Outbound tab. Switch to manual, and then edit rule for LAN2, and choose the .20 IP for the translation address.