Google Contact Sync requires you to use Google oAuth and generate client ID and client secret for your install.
First go to https://console.developers.google.com/apis/credentials and then click create credentials.
then choose OAuth client ID. When setting up your project, choose Web Application and add your domain.
Next you need to enter a redirect URL as below
Being sure to replace https://yoursite.com/ with your site information.
You’ll also need to setup the oAuth consent screen. This is what you are redirected to when clicking to authenticate Google.
Finally, the domain verification so that you only give access via an approved domain.
Once all this is set up, you’ll be given your client ID and client secret.