You can find an extended Documentation of the Gateways incl. Software Updates in the Kerlink Wiki. To get an access, you need to contact the kerlink support team.
In case you want to try out the Login on your own, you can try the standard root password ("pdmk-0b"), which needs to be combined with the last 6 digits of the Mac-address (=serial number of the Gateway). But based to the software version on the Gateway, this password may have to be written in different ways (lower/upper case).