Embedding Youtube video on guestbook

No, not really, sorry.

You would need to be able to add a shortcode to the entry in the bbcode menu.
Then have a plugin that makes that shortcode into a youtube video.
And then you would need to hook into the guestbook plugin with code like this:

function my_gwolle_gb_entry_read( $output, $entry ) {
        $output = do_shortcode( $output );
        return $output;
}
add_filter( 'gwolle_gb_entry_read', 'my_gwolle_gb_entry_read', 10, 2 );