Hi, sorry for the late reply.
If you want to use the spanish translation (es_ES) you should set
define (‘WPLANG’, ‘es_ES’);
in your wp-config.php and the plugin should appear in spanish! For further information: http://codex.wordpress.org/Installing_WordPress_in_Your_Language
The plugin has a spanish translation, but it might be old.. you can find all files in the /wp-content/plugins/wp-gpx-maps/languages folder
If it’s still not working you can check your qtranslate setting and set es_ES as default language..