WordPress 4.7.1 introduced a bug that can cause this error. We have included a workaround for this in Maps Marker Pro v4.3, so updating to the latest version should fix the problem.
If you are on v4.3 or later and still get this error, it is most likely a problem with the GPX file itself. WordPress checks if the contents of a file match its reported MIME type and rejects the upload if they do not. Since GPX files are technically XML files, a valid XML header needs to be present. This means when you open the file with a text editor, the first line should look similar to this:
<?xml version="1.0" encoding="UTF-8" ?>
If that is not the case, adding the line should resolve the error.
Alternatively, although not recommended for security reasons, you can disable the MIME type check by adding the following code to your wp-config.php:
define('ALLOW_UNFILTERED_UPLOADS', true);
If you are still experiencing issues, please open a support ticket and attach your GPX file.