This is a very cool solution and I will play with it soon, thank you!
I read this topic and learned there are some limitations to consider. For example, Google Sheets allows 5 million rows and the form responses go against your daily email limits depending on your free or paid (G Suite) accounts.
Those numbers are pretty high and I assume if you have a blog that big, you may not be using Jekyll or would at least want to use another service provider that solely focuses on commenting. Just thought I would share that this could be a concern for some people with more popular blogs.
Thanks again!