Since a couple of days/weeks, the rendered map is not updated when I modify the GPX file. The Track Editor works fine, I can check via SSH or HTTP that the file has changed.
It seems that Google doesn't reload the file when its URL remains unchanged. If I rename the file, the rendered map gets updated.
I know we shouldn't modify the plugin source code, but here is a quick workaround :
/wp-content/plugins/google-maps-gpx-viewer/google-maps-gpx-viewer.php, line 90 :
$gpxfile = htmlspecialchars_decode($attr['gpx']);
$gpxfile .= '?v='.rand(); // Force Google to download the file
Maybe you could include it in the next release ?