You can use wordpress as your CM. Write posts and they appear in your Gatsby site. Contentful is probably a better option though...
also, wordpress builds tend to also take the longest.
if you enjoy hacking on wordpress and debugging weird issues, then this might be a good choice.
It's much easier to build a blog using prismic, contentful, or even the netlify cms kind of is nice for a basic "serverless" blog/cms.
honestly, i'd take markdown files in a bunch of folders any day over gatsby-source-wordpress.