Google App Engine(GAE) is a Platform as a Service(PaaS) offering on Google Cloud Platform, which abstracts away the infrastructure so that you focus on your web application code. App Engine handles both automatic scaling up and down of instances on-demand for your web application based on the number of requests.
App Engine is available in 2 types of environments - Standard and Flexible and supports the following programming languages: Go, Java, Python, PHP, Node.JS, Ruby, and .NET.
Using the Users API is a very simple and flexible way to work the authentication in App Engine, but please make sure that your application cases don't require more elements for the authentication environment.
Note: If you need more information about the traditional structure of an App Engine app, please review this info.