You said: The issue is when I try to set the gateway for the WAN interface using the webGUI "add gateway"
I do not understand that part.
As soon as a WAN interface is set up (Interface => Assign) and it's UP (Status=> Interfaces) it will be listed in Status => Gateway.
I'm not able to add a gateway using Interfaces => WAN - WAN is a gateway.
Btw: I have only one WAN interface, do you have more then one ?