Dear Tor
In dial up gateway/server mode your actual gateway IP is generally provided automatically by your ISP so you cannot set it to anything else than what it is, note your local IP is different to your gateway IP.
If you have a second server (or other device) on your network that is acting as a gateway you can tell your first server to get its internet access from (via) the second server (or device), this is the gateway IP setting that you would change on your first server.
You change this in the Server Console, Configure this server option after you have logged on to your server as admin..
Step through the options till you get to "Select master DNS server address", where it says quite clearly "You may need to specify an IP address here if there is a firewall between this server and the Internet.
Firewall=gateway
This is where you enter the local IP of the second server (or device) that is acting as a gateway for your first server.
Please read all the screens carefully, it all makes sense if you take the time to absorb what the instructions are saying to you.
Hope this helps
Regards
Ray Mitchell