Use "save configuration <filename>" on your source GW to save OS configuration settings to a file. Copy out the file and go over it, primarily to check that interface names are the same as on the new appliance. If it is not the case, you will have to edit the interface names to use naming convention of your new appliance.
Finally, copy the file to the freshly installed new appliance and perform the following:
HostName> set clienv on-failure continue
HostName> load configuration <filename>
HostName> set clienv on-failure stop
HostName> save config