It's surely doable. Gatsby is gaining momentum and I'm amazed how fast it is. Your whole architecture is fine, but you should take some notes into consideration:
While almost everyone can add a new page or make a small change to a WordPress's site using the admin panel, this is not the case with static site generators. At least a basic knowledge of using git or Github is needed to make the smallest change.
Of course this can be avoided if you extend the functionality and grant write control to almost all files using a web interface or another GUI, but that involves a lot of additional extension and coding.
GitHub Apps use the installation's minimum rate limit of 5,000 requests per hour. For a relatively small to medium site that's ideal, but you need to prevent any abuse of the system. For example, if you use GitHub's API to search the content, a malicious user can exploit the search and waste your requests, breaking the normal workflow for the whole site. Another solution to that problem would be to search on the client side.
As of today, I haven't found a reliable admin panel for Gatsby (or any static site generator). That means you need to use a third-party service or create it by yourself. The last can become a really time consuming task.
That being said, you should definitely try Gatsby or any other static site generator. The fast deployment, the overall speed and the flexibility of the whole system at a cost of around 0$ is like a dream come true.