Both of our API frameworks have been open sourced (Java[0], Python[1]). Those used to be available only on App Engine Standard Environment but can now be used anywhere. This was a top request of people using Flexible Environments.
But we've added a bunch of functionality to the platform (logging, monitoring, user authentication, tracing) that people can use whether or not they're using our frameworks -- for example, Python+Flask, NodeJS + Express, etc. To use it, you set it up with an OpenAPI Specification (FKA Swagger Spec). We've seen tons of interest in OpenAPI and even joined the Open API Initiative to foster that community.
If you are interested in a walkthrough (that uses Python and Flask), see [2].
[0]https://github.com/cloudendpoints/endpoints-management-java [1]https://github.com/cloudendpoints/endpoints-management-pytho... [2]https://cloud.google.com/endpoints/docs/quickstart-app-engin...