Problem with configuring Netlify CMS with Git Gateway

I am trying to use this Gatsby starter with Netlify CMS. https://github.com/stackrole-dev/gatsby-starter-foundation

I followed the instructions exactly but after enabling Git Gateway, when I try to login in as admin I encountered this error massage.

Your Git Gateway backend is not returning valid settings. Please make sure it is enabled.

I have no ideas why it is not working.

My config.yml is

backend:
  name: git-gateway
  commit_messages:
    create: 'Create {{collection}} “{{slug}}”'
    update: 'Update {{collection}} “{{slug}}”'
    delete: 'Delete {{collection}} “{{slug}}”'
    uploadMedia: '[skip ci] Upload “{{path}}”'
    deleteMedia: '[skip ci] Delete “{{path}}”'

local_backend: true # run npx netlify-cms-proxy-server for local testing

media_folder: "static/assets" 
public_folder: "/assets" 

collections:

You need to enable your settings for git-gateway and external providers in your Netlify as shown in Netlify documentation:

This configuration can be found under https://app.netlify.com/sites/YOURNAME/settings/identity

In addition, your config.yml lacks:

backend:
  name: git-gateway
  repo: username/repository
  branch: master

Note: change username and repository for your names.