Do you want to know what SMTP gateway you should reference? Or are you actually trying to determine the address of the gateway?
You should be able to use the PING command at a command line to see what the IP address is. Just PING the name of the server and the IP address should be shown in the output.
But that doesn’t make a lot of sense to need to know.
If you only want to know the name of the SMTP gateway that you should be using, you need to ask your ISP or ask whoever configures e-mail for your organization.