Hi Alessandro,
When you say 'Allow new people to sign up as guests', I'm guessing that you want new users to have an account and not just be logged in as a guest, right?
You can disable all login possibilities except Google OAuth by just enabling OAuth 2 authentication and setting up the OAuth 2 Google service only.
You can then choose whether new people will have an account created for them automically or not, by disabling or enabling the setting 'Prevent account creation when authenticating' in Site admin / Plugins / Authentication / Manage authentication.