The HP 7470 is an HPGL plotter. To use it as a native plotter from any OS you need the appropriate driver. There's a driver for Win9x but not for Win 2k, NT or XP, so you can't use any HPGL device in the later OS's native.
Autocad can drive the plotter directly from any OS by setting up a custom plotter under autocad and specifying the port and connection. The autocad 2000 help is pretty good at explaining it.
To plot G-code on the plotter you have to convert it to HPGL. You could import your Gcode into a DXF in autocad directly, a contributer here has posted an autocad program to do this, and then plot the dxf to plotter with autocad managing the HPGL side. I have done this with my Mutoh F920 which is a HPGL only plotter of the same vintage.
Note that the conversion process might change the output somewhat, or miss some codes entirely.
If you smply want to see the actual product of your Gcode I'd recommend CNC Simulator. Its a free software that does a realtime toolpath emulation on the PC. Saves Paper !
Do a websearch for a Gcode interpreter to convert G-code to HPGL. Several machines and manufacturers have used HPGL to operate machines and input Gcode and you could then print the file directly.