Google Docs keeps a detailed history of each doc, which tracks the changes and can restore previous versions later, just like Git.
doc2git lets you create an actual Git repo from this history, with a plain text version of your doc and a commit for each revision with the correct time and author.
With the Git repo, you can use all your usual tools to examine the doc history, including git blame.
It all runs securely in your browser. Even the Git repo is created client-side using isomorphic-git and delivered as a .zip file you can download.
You can check out the code of doc2git on GitHub, including instructions on how to host doc2git yourself.
Sign in with Google
First sign in with a Google account which has access to the Google Doc:
Enter the Google Doc URL or ID
Download the repo