From dmfswiki
Who would want to use CardDAV-Sync or CalDAV-Sync with a Google-Account? Every device should do that natively. However if you want to use CalDAV-Sync to sync your Google Calendar enter the following data.
Setup
CalDAV-Sync
To set up a CalDAV account press Manual Mode first.
- CalDAV URL: https://www.google.com/calendar/dav/
- Username: your Google Mail address
- Password: your Google password
Currently one-way-sync only. Uploading events seems to be broken (go to http://code.google.com/p/google-caldav-issues/issues/detail?id=26 to see why).
CardDAV-Sync
CardDAV-Sync is untested. Does Google support CardDAV at all?