Exclude post thumbnail from wordpress native gallery

exclude_thumbnail_from_gallery.php

<?php
function exclude_thumbnail_from_gallery($null, $attr)
{
    if (!$thumbnail_ID = get_post_thumbnail_id())
        return $null; // no point carrying on if no thumbnail ID

    // temporarily remove the filter, otherwise endless loop!
    remove_filter('post_gallery', 'exclude_thumbnail_from_gallery');

    // pop in our excluded thumbnail
    if (!isset($attr['exclude']) || empty($attr['exclude']))
        $attr['exclude'] = array($thumbnail_ID);
    elseif (is_array($attr['exclude']))
        $attr['exclude'][] = $thumbnail_ID;

    // now manually invoke the shortcode handler
    $gallery = gallery_shortcode($attr);

    // add the filter back
    add_filter('post_gallery', 'exclude_thumbnail_from_gallery', 10, 2);

    // return output to the calling instance of gallery_shortcode()
    return $gallery;
}
add_filter('post_gallery', 'exclude_thumbnail_from_gallery', 10, 2);
?>