Not completely sure I understand the question, but I can elaborate on the differences between the modules and maybe that will answer it.
The original ECOM did not ever generate its own traffic...like sending an email...it simply responded to requests from clients. As such, it did not need a gateway address...it simply responded to whatever address sent in the request. In a routed environment, the gateway is always the guy on the local network that is forwarding the request, consequently, by answering him you are always answering the right node.
With the ECOM100, it implements a full TCP/IP stack and is capable of initiating traffic, consequently it needs to know the address of the gateway to be able to send requests outside the local network. Since the ECOM100 stack has gateway awareness, it needs a valid gateway to function correctly.
Not sure if that helped...but if not, I can try again.