BT3 is designed from the ground up as a Penetration Testing toolset, and as such is NOT made to be a gateway or nework monitor. Like any other Linux distro, you could probably add a lot of things to BT3 to get it to act as a gateway, but it would require a llot of time and effort to do that, and would be a waste of the installed tools, which aren't designed for a gateway or monitoring machine.
There are other distros designed from the ground up to perform these functions. First, however, you really need to determine which function is more important to you, the network monitoring or the gateway. The two are really separate functions.
Smoothwall is a great Linux distro that is a gateway, and has some limited monitoring built-in to it. This is what I use. The administration is completely web-based.
Cacti is a great Linux network monitor, but has no real gateway functions. A nice variant is EZCacti which is a complete ISO image, and it great for starting your first monitoring station. Streaker69 has used Cacti extensively, and loves it for his WAN and multiple LANs.