Problem on terminate to a Registered gateway

Sorry we are a newbie in FreePBX and voip.

We have 2 ICS PowerGate 16V GSM-VoIP Gateway ( I configued it with public IP and as a gateway no register to my FreePBX, it works fine on outgoing call for gsm termination as a trunk. The FREEPBX could send the call to the gateway directly as peer to peer mode.

But now, we want to move the PowerGate to the LAN (in NAT), so we configued it to register to the FREEPBX with account/pass as 1007/1007. It registered successfully.

But my question is how could we send the call to a registered gateway in FREEPBX? Does any one could share a demo configuration on how to send a call to a Registered termination gateway for the FREEPBX?

Thanks a lot.