Hi Mauro,
(sorry the late reply, i have been on holidays)
I would always recommend providing the ZIP file option.
If you want the plain GPX file available for download, you can add this line:
AddType application/octet-stream .gpx
to your .htaccess file on your server.
I found this here:
css-tricks.com/snippets/htaccess/force-f...not-open-in-browser/
In HTML5, there would be an alternative solution, namely to use the new "download" attribute for the a href link. Unfortunately this does not work in all browsers (specifically Safari doesn't support it), so the .htaccess solution looks better to me.
cheers, Frank