firewall-two gateway problem

I have a firewall question that I hope someone can answer for me. The
problem revolves around two gateways (server A and server B).

Currently I am using an old linux box as a firewall (server X) and
everything works but it is an old machine and needs to be replaced. So I
have set up a new firewall (server A).

A normal firewall setup has the nodes on the LAN use the internal IP of
the firewall (server A) as their gateway. My problem is that the nodes
on the LAN are using server B as their gateway. Currently the system
works fine but as I mentioned I am trying to replace the existing
firewall (server X) with server A.

I have set up a test computer outside the firewall (server A) and a test
web server behind the firewall with its gateway set to the IP of the
internal NIC on the firewall (server A). Then I have firewall rules set
up to allow port forwarding to the internal test web server.

This scenario works flawlessly but when I set up the firewall rules to
allow port forwarding to a server on the LAN that uses server B as its
gateway it does NOT work. It's obviously a routing problem.

No settings are changed on the servers on the LAN. It's all done on the
firewall but I can't figure out how.

Can anyone offer any insight into this?