Add Sitegeist.GoogleAuth/Configuration/Routes.yaml to your Neos/Configuration/Routes.yaml
Add Sitegeist.GoogleAuth/Configuration/Settings.yaml credentials to your Neos/Configuration/Context/Settings.yaml
Visit Google Developer Console:
https://console.developers.google.com/apis/credentials?project=weblogin-oauth&authuser=0&pli=1
Add authorised redirect URL to oAuth 2.9 Client ID:
Neos WebLogin oAuth