add_filter(
    'sp_wpcp_image_attachments',
    function( $attachments, $shortcode_id ) {
        // your shortocde id
        if ( '1252' === $shortcode_id ) {
            $gallery     = get_field('gallery');
            if ( is_array( $gallery ) ) {
                $attachments = $gallery;
            }
        }
        return $attachments;
    },
    10,
    2
);