All do give the correct first few steps;
For Debian flavored distros:
sudo apt-get install akonadi-kde-resource-googledata
for RPM based distros, using yum:
yum install akonadi-googledata
Next they tell you to Select Akonadi Google Calendar Resource. This is incorrect, you should actually select Google Calendar and Tasks.
The following window will appear when you do;
Your's of course will be empty. Click on the Add button at the top to setup access to your Google services. In the next dialog box enter your Gmail address and password.
Lastly you need to accept the following so you can get your calendar and tasks loaded into Kontact;
That's it, your done and Kontact now has access to your Gmail services.