For OAuth2, the documentation is here: https://pusher.github.io/oauth2_proxy/
You'll need to follow the Auth Configuration part first on the google site. Make a note of your client id and secret.
Next, you need two files in your /appdata/oauth2 folder.
emails.cfg - a file with the allowed authentication emails in, one per line.
EMAIL@gmail.com
EMAIL@DOMAIN.org
oauth2_proxy.cfg - example below seeing as the example in the docs is broken.
## OAuth2 Proxy Config File
## https://github.com/cheesemarathon/oAuth2