import post images to acf gallery

importer.php

<?php
// field id must be the ACF ID
function post_images_to_ACF_gallery($post_type, $field_id){
    $args = array(
        'post_status' => 'publish',
        'post_type' => $post_type,
        'posts_per_page' => -1,
        'fields' => 'ids'
    );
    $the_query = new WP_Query( $args );
    if ($the_query->have_posts()) {
        while ( $the_query->have_posts() ) {
            $the_query->the_post();
            $images = get_all_post_attachments($the_query->post);
            $ids = array();
            foreach ($images as $i) {
                $ids[] = $i->ID;
            }
            update_field($field_id, $ids, $the_query->post);
        }
    }
}

function get_all_post_attachments($id){
    $images = '';
    $g_args = array(
        'post_parent' => $id,
        'post_status' => 'inherit',
        'post_type' => 'attachment',
        'post_mime_type' => 'image',
        'order' => 'ASC',
        'orderby' => 'menu_order'
        );

    $attachments = get_children($g_args);
    return $attachments;
}