Create a new app at: https://console.developers.google.com (for more details visit: https://developers.google.com/identity/sign-in/web/devconsole-project )
Go to https://console.developers.google.com/apis/credentials?project={your app name}
Set Authorized redirect URIs to {your site url}/DesktopModules/DnnSharp/Common/WebHandlers/DnnSharpGPlus.ashx/
Side note:
It might take a few minutes until the the login will actually work.
Where to find Google App ID and App Secret:
Create a new app at: https://console.developers.google.com
Go to https://console.developers.google.com/apis/credentials?project={your app name}
Side notes
The Gplus Login action also creates the GplusId profile property. And stores the created users Gplus client id in it.