Obtain a Google OAuth2 application client id and secret from Google Developer Console. Navigate to APIs & Services, then Credentials. Add http://localhost:8080/google/callback as a valid OAuth2 Redirect URL.
main.go shows an example web app that issues a client-side session cookie. Pass the client id and secret as arguments or set the GOOGLE_CLIENT_ID and GOOGLE_CLIENT_SECRET environment variables.
go run main.go -client-id="692352861178-8gqs4oebvvsl5bmmb85kju5bl5739tdq.apps.googleusercontent.com" -client-secret=secret