When you connect to the internet with a Router, the Router *IS* the Default Gateway for any computers connecting to the internet through it.
>>> I have tried to split this before by plugging
>>> the ethernet into an old hub (netgear dual
>>> speed hub DS104). Unfortunately when I tried
>>> to run two PCs from the Hub when the 2nd
>>> Connection was made the 1st connection was dropped.
I would have thought that should work, although it might be necessary to use a Network Switch rather than a basic Network Hub. The way to check would be to see what IP address each computer connected to it gets by DHCP. If the first computer to connect gets your public IP address (as allocated by the ISP) then you are connected to a Modem and not to a Router. A Router will get your public IP address and allocate IP addresses to connected computers using one of the address ranges reserved for Local Area Networks, most commonly 192.168.x.x addresses.