To use the Drupal 7 content on this Gatsby site you have to export it first as Markdown.
There are two scripts, one for exporting nodes and the other for exporting users.
node-migrate.php user-migrate.php
The files have to be placed on the root of the docroot folder of the original Drupal 7 website and execute them from a fully functional environment.
To export users
php user-migrate.php
To export nodes you have to specify the node type.
php node-migrate.php [node type]
For example for blog posts.
php node-migrate.php blog
Users and content will be exported to the static folder on the root of the project.