Download "Ride Leader" by Trail Tech. It is free. It will open gpx and kml files. Kmz(Google Earth) files are just zipped versions of Google Earth (kml) files. Google Earth will open both formats. Open the gpx file with Ride Leader then save it as a kml (Google Earth) file. You can then open that file with Google Earth. It is very easy. Let me know if you have any questions.
Have a Blessed Day,