Normally you would know an IP-address of the gateway on shipment or from referencing WIKI.
However, what if you are getting back to the device sometimes later after certain changes was made?
One of the ways to acquire device setting is browsing system USB-flash on some computer.
The system USB-flash is formatted as ReiserFS so you can read it anywhere just by installing supporting software. Of course, you should avoid re-formatting USB-flash and performing any disk-checks, as may be advised by some host OSes. You absolutely need to keep it as original. Further, with the help of certain Win/Mac/Linux utilities you can reach any files for reading.
To find an IP-address of the GSM-gateway you navigate to /etc/rc.d/rc.network and browse the rc.network file until you see "echo LAN mode" text. This section lists your current network settings.
The same way you can jump to config-files of native SIP-client (SipUA) as well as to Asterisk main config-files such as extensions.conf and sip.conf.