One more thing before I peace out for the night. I read all the questions and comments on the Hackster page that Gus posted to. I added this post to hopefully help noobs like me. Its basically the same thing as I noted above but thought it could not hurt to post it here as well.
I wanted to share my experience with this. It has been thrilling and incredibly frustrating. In my experience, if you are getting “undefined” in your columns switch your webhook from JSON to Web Form, or from Web Form to JSON and enter the same parameters. I have two identical apps with identical webhooks. Only the names are different. One will post to sheets with JSON, the other only Web Form. Also if you are having trouble with the google script app not testing correctly try pasting the code again and running through the setup process. This has happened to me every time and I find if I just past the code over again it fires right up. Good luck and keep at it. Cheers.