Unfortunately, you cannot convert kml to gpx just right in Google Earth. However some tools like GPSBabel can easily convert various GPS formats between each other, which also includes as kml to gpx conversion. Used to transfer map data.
Open KML file Open GPX fileA GPS mapping tool
Program for Linux that allows users to convert waypoints, tracks, and routes between popular GPS receivers
An online GIS converter
A simple converter for OziExplorer calibration files, tracks and points