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