You don't need to define a gateway if there is only one Internet connection. Under the LAN's DHCP settings, make eBox the gateway. Outside the DNS stuff, that's pretty much it.
I apologize for not being near an eBox so I can write up a quick step-by-step. The basics, from memory, are:
*The DCHP and Network modules need to be enabled.
*One NIC needs to be set as external and have its address assigned by DHCP.
*One NIC needs to be assigned a static address (e.g., 192.168.1.1 if the LAN will use the 192.168.1.0/24 address space).
*Under the DHCP settings, make eBox the default gateway (as described
here).
Under this basic setup, there is no need to define a gateway (where, as you noted it is currently necessary to have a static address for the external interface). Ignore that section of the Network setup.