Google calender available at http://calendar.google.com/ for gmail users.
Kontact/KOrganizer can use to read the calender created in Google.
In Google Calender
Go to settings → Calenders → [name of calender]
In private address, click on the ICAL icon . A small dialog will pop up, copy the URL given.
In KOrganizer, click add resource, select Calender in Remote file.
Choose a name for the resource.
Paste the above URL in Download from area.
Check on „On Startup“ in Automatic Reload
Check on „On every change in Automatic Save
Click Ok, and you are done setting up calender in Kontact.
Uploading or event modification wasn't possible for me, sounds like google calender does not support uploading of calenders, yet.
I found a way to export from KOrganizer to Google Calendar, however it has to be done manually each time.
In KOrganizer: File –> Export –> vCalendar (You can try iCalendar, but it didn't work for me. vCalendar doesn't seem to do repeating events too well though, just so you know). Save that file somewhere.
In Google Calendar: Go to Settings –> Import Calendar.
The rest should be pretty easy….
Just use GCALDeamon: http://gcaldaemon.sourceforge.net/