It has been a vry long time since I worked with HP Basic but a GPIB Read should work with instrument assuming that there's data in the buffer, you have the correct address, the instrument's connected, and you're using a NI GPIB card. Have you tried something simple like writing IDN? and then doing a read? Does the instrument show up in MAX and can you communicate with it there?
I would also suggest that you use VISA instead of the traditional GPIB functions. They give you some more flexibility.
