class JDN_Create_Media_File {

    var $post_id;
    var $image_url;
    var $wp_upload_url;
    var $attachment_id;

    /**
     * Setup the class variables
     */
    public function __construct( $image_url, $post_id = 0 ) {

        // Setup class variables
        $this->image_url = esc_url( $image_url );
        $this->post_id = absint( $post_id );
        $this->wp_upload_url = $this->get_wp_upload_url();
        $this->attachment_id = $this->attachment_id ?: false;

        return $this->create_image_id();

    }

    /**
     * Set the upload directory
     */
    private function get_wp_upload_url() {
        $wp_upload_dir = wp_upload_dir();
        return isset( $wp_upload_dir['url'] ) ? $wp_upload_dir['url'] : false;
    }

    /**
     * Create the image and return the new media upload id.
     *
     * @see https://gist.github.com/hissy/7352933
     *
     * @see http://codex.wordpress.org/Function_Reference/wp_insert_attachment#Example
     */
    public function create_image_id() {

        if( $this->attachment_id )
            return $this->attachment_id;

        if( empty( $this->image_url ) || empty( $this->wp_upload_url ) )
            return false;

        $filename = basename( $this->image_url );

        $upload_file = wp_upload_bits( $filename, null, file_get_contents( $this->image_url ) );

        if ( ! $upload_file['error'] ) {
            $wp_filetype = wp_check_filetype( $filename, null );
            $attachment = array(
                'post_mime_type' => $wp_filetype['type'],
                'post_parent' => $this->post_id,
                'post_title' => preg_replace('/\.[^.]+$/', '', $filename),
                'post_content' => '',
                'post_status' => 'inherit'
            );
            $attachment_id = wp_insert_attachment( $attachment, $upload_file['file'], $this->post_id );

            if( ! is_wp_error( $attachment_id ) ) {

                require_once( ABSPATH . "wp-admin" . '/includes/image.php' );
                require_once( ABSPATH . 'wp-admin/includes/media.php' );

                $attachment_data = wp_generate_attachment_metadata( $attachment_id, $upload_file['file'] );
                wp_update_attachment_metadata( $attachment_id, $attachment_data );

                $this->attachment_id = $attachment_id;

                return $attachment_id;
            }
        }

        return false;

    } // end function get_image_id
}


$gravity_form_id = 1; // gravity form id, or replace {$gravity_form_id} below with this number
add_filter( "gform_after_submission_1", 'jdn_set_post_acf_gallery_field', 10, 2 );
function jdn_set_post_acf_gallery_field( $entry, $form ) {

    $gf_images_field_id = 3; // the upload field id
    $acf_field_id = 'field_5de3b8157dffc'; // the acf gallery field id

    // get post, if there isn't one, bail
    if( isset( $entry['post_id'] ) ) {
        $post = get_post( $entry['post_id'] );
        if( is_null( $post ) )
            return;
    } else {
        return;
    }

    // Clean up images upload and create array for gallery field
    if( isset( $entry[ $gf_images_field_id ] ) ) {
        $images = stripslashes( $entry[ $gf_images_field_id ] );
        $images = json_decode( $images, true );
        if( !empty( $images ) && is_array( $images ) ) {
            $gallery = array();
            foreach( $images as $key => $value ) {
                // NOTE: this is the other function you need: https://gist.github.com/joshuadavidnelson/164a0a0744f0693d5746
                if( ! class_exists( 'JDN_Create_Media_File' ) )
                    break;

                // Create the media library attachment and store the attachment id in the gallery array
                $create_image = new JDN_Create_Media_File( $value, $post->ID );
                $image_id = $create_image->attachment_id;
                if( absint( $image_id ) ) {
                    $gallery[] = $image_id;
                }
            }
        }
    }

    update_field( $acf_field_id, $gallery, $post->ID );
}