Hello JP,
The recommended user authentication mechanisms for App Engine Standard Environment and App Engine Flexible Environments are Firebase Authentication, Google Sign-In, OAuth 2.0 and OpenID Connect as outlined in these documents [1][2]. For App Engine Standard Environment it is also possible to authenticate users using Users API.
This document provides information on how to get a user's identity with Identity-Aware Proxy (IAP). By using signed headers or by using the App Engine standard environment Users API you can get the IAP-authenticated user identity.
This document provides further information on the Consent screen.
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