Think of the GPS/NAV selector switch as 2 different sources from which the aircraft uses to navigate.
GPS= follows the GPS flightplan, wpt after wpt
NAV= radio navaids. Use this to fly a VOR radial or Localizer course (ie ILS approach)
Now the actual NAV button on the autopilot engages the SELECTED nav mode (GPS or radio navaid, remember

)
So if you want to fly the whole flight plan automaticly you will choose GPS as navigation source and then push the "NAV" button on the autopilot panel.
But if you want to fly VOR-VOR for example, you must select NAV instead of GPS, tune in your desired frequency in the radio stack and course in the "course" window and then push the NAV button on the AP panel
I know this is confusing in the beginning, but hang in there buddy :)