https://forestry.io just launched beta support for Gatsby (like today). Forestry.io is a CMS for markdown + Git, where all edits get committed to your repo.

If you want access to the beta, message the folks at support@forestry.io.

We've been recommending Contentful and DatoCMS (https://www.datocms.com/) to our customers. Most pick Contentful due to the perception of being enterprise (Replacing AEM, Sitecore etc.). DatoCMS seems to enjoy a lot of warm reception by people who are coming from Squarespace/Wordpress/Drupal and want something headless but still geared towards Web.

There's also Sanity, which supports self-hosting of the app itself and is mostly open source:

https://www.sanity.io

(Disclosure: I'm involved in the company, but not in a major way.)

Here is a long list of headless CMS

https://headlesscms.org/

Contentful is also a good option.