Jonathan,
You're new - no biggie, that's what the forums are for. To answer your question, you do not need to register the public IP address of the gateway in order for it to receive calls. As Java said, this is an analog gateway so there is no use case for doing so. In fact, this would probably never be necessary.
On to other things. I would strongly recommend against having public IP addresses assigned to your voice infrastructure unless you are well protected from unauthorized external access. There are some that would argue public vs. private addressing doesn't really matter. I am of the latter opinion unless there is some special requirement or use case that must be accomodated in this fashion. In either case, you still need to be protected from external attack (in general).
As for when the gateway IP may be important, that comes into play more internally - i.e., for internal communication between the voice infrastructure devices and the phones. As for external entities, this might come into play if you were doing some sort of inter-company SIP configuration; however, even then most things would flow through a proxy server and, in the end - callers would still dial your end users by number (i.e., they are blind to where the call goes other than knowing the person answers on the other end).
HTH.
Hailey
