firewall help

[freejay]

Hi, currently our network (2.2.2.x/16) is connected to a gateway router
(with an IP address of 2.2.2.1) and we do not have any internet connection.
My boss told me that our network would be connecting to the internet soon,
so I better build a firewall for this. The firewall we came up with is a 3
interfaced linux box. One interface is connected to the internet, another to
our network, and the last one to the gateway router. Here's the problem:
1.) the firewall should be transparent enough between the gateway and our
network that if we need to disconnect from the firewall we could connect
directly to the gateway router.
2.) Our current settings on both gateway and network is retained even if
connected to the firewall. (e.g. the gateway of our PC's are still 2.2.2.1)
Hope somebody can help.