Just a few links to some of the open tabs from when I was adding tag support to my Gatsby site.
https://github.com/erikthedeveloper/erikaybar.name-gatsby/blob/master/src/pages/index.js
https://github.com/erikthedeveloper/erikaybar.name-gatsby/pull/1/files
https://www.gatsbyjs.org/docs/adding-tags-and-categories-to-blog-posts/
And another tab/link that I still had open (from some initial setup effort): https://www.gatsbyjs.org/tutorial/part-zero/