It was most likely using the GPS-OBS system, to select the runway (or less accuratly the airport) as a waypoint, and then use OBS to set a magenta bearing line on the runway heading,
You can then fly a heading to intercept that Magenta line, (heading mode) switching to Nav Mode when the CDI has a 75% or less deflection, and the plane will lock on and follow the OBS Magenta Line TO the runway. You will have to handle your own decent, and stop using the GPS when down to the Minimum GPS AP permitted altitude,