allow users to upload gpx files

What type of file is gpx? What application uses it?

GPX is https://filext.com/file-extension/GPX

It’s a gps exchange format.

Something like this should do (not tested but this should cover possible mime-types for GPX (at least the ones I found googling)):

<?php
add_filter( 'fu_allowed_mime_types', 'hannekemijnklim_fu_allowed_mime_types' );
function hannekemijnklim_fu_allowed_mime_types( $mime_types ) {
    // GPX can use different mime types
    // Here we use gpx, gpx2 etc to differentiate the keys
    $mime_types['gpx|gpx1'] = 'text/xml';
    $mime_types['gpx|gpx2'] = 'application/xml';
    $mime_types['gpx|gpx3'] = 'application/gpx';
    $mime_types['gpx|gpx4'] = 'application/gpx+xml';

    return $mime_types;
}

you can also add add_filter( 'fu_is_debug', '__return_true' ); and that will show you mime type of a file you’re trying to upload

Unfortunately it isn’t working.

When I try to upload a gpx it said: Your submission failed spam checks

Do you know how I can fix that?

disable akismet and recaptcha in settings and try again.