Not possible, the gateway is the internal-facing NIC. The only thing a gateway IP does is provide a MAC address to use for the destination of your off subnet bound traffic, which would be the same no matter how many IPs you put on an interface. If you dual homed the source machine, and put two internal NICs in the firewall, and could control routing from the source OS, you can do it that way. In most circumstances, such routing from the source OS will not be feasible.