Special characters in URLs have to be "url encoded" before passed to the browser. See http://www.w3schools.com/TAGS/ref_urlencode.asp
for example and test.
BUT: it is anyway a very very bad idea to call a script (or file) "Gästbook", as it is impossible to determine which character set is used on the webservers file system (utf8, latin etc.). Due to that problem a HTTP Request for a filename including such special chars may fail anyway, even if the URL seems to be encoded correctly.
Therefore you really *should* rename your script to "guestbook.php" or "gastbok.php" or similar. Otherwise some browswer will successfully open and others will fail to open that file. The behaviour is unpredictable.