AndreasR
The problem with the tutorial you linked is that it is outdated and does not work.
There is information in the tutorial which does not exist but is required...
If you can find me the: 'Client Secret', I will stand corrected.
Here is the quote from the tutorial entry:
"Client secret
The client secret for the game from the Google Play Developer Console. This is only necessary when publishing via Cordova (e.g. PhoneGap Build). For more information see the section Setting up Google Play Game Services above."