When Netlify CMS uses the git-gateway backend, it will check for a
window.netlifyIdentity
object and use that to handle the whole auth flow.
This also sets defaults for the git-gateway endpoint, that means it
can be used in templates with zero configuration and fit with a
one-click deploy to Netlify approach.
Netlify Identity itself is based on our open-source GoTrue microservice,
and Netlify's Git Gateway project is completely open-source as well.
The git-gateway backend will work with Netlify without any setup, but can
also be configured to work with any selfhosted GoTrue and Git Gateway
instances.