To sum up: Netlify CMS is not meant to do what you’re trying.
Longer version: You’re choosing to ignore all that I said before. Netlify CMS is meant to enable adding/editing posts which you can do by visiting
/admin and it seems like you already know this. You even have this configured, but you’re fixated on being able to add a page at
/blog by just adding Netlify CMS. This is not what Netlify CMS is meant to do or can do. You could create a new page from within the CMS to be available at
/blog, but that’s not what you’re trying to do. What you’re looking for is to add a page at the
/blog path and to do this, you need to configure your SSG (Gatsby). You need to create a page in it and add a logic to generate some content to show there.