Information can be found in the following websites:

For using Google Earth directly:
http://earth.google.com/userguide/v4/ug_gps.html#point

Tools like ogr2ogr can convert gpx to kml, will require some playing around.
http://www.gdal.org/ogr/drv_gpx.html
http://www.bostongis.com/?content_name=ogr_cheatsheet

You can convert GPX to shapefile:
http://gpx2shp.sourceforge.jp/

and then shapefile to KML:
http://freegeographytools.com/2007/export-shapefiles-to-google-earth-ii-shp2kml-converter