Currently, only IMAP-based address books allow their respective CardDAV URL to be shown in the web app by using the $config['kolab_addressbook_carddav_url'] variable. LDAP-based address books have this function greyed out and currently do not support it.
The URL for them is constructed with:
/addressbooks/<user-email>/ldap-directory/
Currently, it is not documented how to access the GAL via CardDAV. Using this already existing Show URL feature, would be great way to do it.