Login with Netlify identity not available

I have my Gatsby + Netlify site deployed and the cms plugin is configured like this under gatsby-config.js:

{
  resolve: 'gatsby-plugin-netlify-cms',
    options: {
      modulePath: `${__dirname}/src/cms/cms.js`,
  },
}, 

Https is enabled on the site but I only get the option to login using github and I would need to enable the Netlify identity log in.

Any clue?

I've faced the same issue a few days ago and I felt just like you, there's a lack of information (or is not properly linked between) so finally, I've managed to achieve this.

If I understood correctly, you want to add, besides some others identities, a typical username and password identity, isn't? I'll try to lead you through those options.

First of all, in your config.yml you need to set up the identity gateway:

backend:
  name: git-gateway
  repo: githubUsername/githubRepo
  branch: master

The git-gateway option allows you to use Netlify back-office options. So the next steps are related to customizing those options in your back-office. Following Netlify docs, you need to set and allow the identity going to the Identity tab and click Enable Identity.

Once enabled, you need to add your external providers as the following screenshot shows:

This will add to your git-gateway a login for your providers.

If you are using GitHub (as it seems) I guess you have the API token correctly, if not, please let me know and I will update my answer.

The last step is to invite users and allow them to log in, this configuration is under Identity tab.

All these configurations will show, in your site, under /admin:

If you click that button you'll see the following login modal: