You're not supposed to receive anything back when you send a command. A GPIB instrument will not automatically echo back a command you just sent it. Most instruments do have a separate query command if you wish to use them. As I stated earlier, most of these commands are terminated with a "?" at the end. Section 3.3.2 of the manual lists the supported queries. For example, you would write an RF? and then do a read to determine the frequency setting.
Just like the other instrument, there is a LabVIEW driver available
here. However, queries for instrument settings are not oftern included in drivers. The sim
ple reason for this is that since you are sending the parameters to the instrument, you already know how it is setup and there is seldom any need to do a read back.
For reading the power meter, the only thing I can suggest is that you use the RSNVRS Measure VI in the driver. It looks like the instrument must receive a trigger command first and then the command "N0" is sent. The response requires a bit of parsing to get something. This appears to be a very old instrument as the command set is very non-intuitive and you have to hope that R&S wrote the driver correctly.