Create WordPress Gallery from BuddyForms File Form Element and add the Gallery under the Content

buddyforms_custom_gallery_content_filte.php

<?php

//Add a content filter to attache your Gallery.
add_filter('the_content', 'my_custom_gallery', 10, 1);
function my_custom_gallery($content){
    global $post, $buddyforms;

    if(is_admin())
        return $content;

    if(!isset($buddyforms['buddyforms']))
        return $content;

    // check if the post is attached to a BuddyFormsm Form
    $form_slug = get_post_meta($post->ID, '_bf_form_slug', true);

    if(!$form_slug)
        return $content;

    if(!isset($buddyforms['buddyforms'][$form_slug]))
        return $content;

    // get the form element (custom field) values
    $gallery = get_post_meta($post->ID,'FORMELEMENTSLUG', true);

    if(!$gallery)
        return $content;
    
    // create the WP Gallery ShortCode and return it
    return  '[gallery ids="'. $gallery .'"]'. $content;
}