I'm not sure this is a secret, I think I've seen people talk about this in the past. I even did a little bit of work looking into whether or not it was feasible as a replacement for CMS backends like Wordpress, but eventually gave up because:

A) Git operations on both Github and Gitlab are kind of slow (compared to normal AJAX requests).

B) It requires you to embed Git credentials into your webapp clientside, and I'm more comfortable building secure client sessions than I am messing around with libsodium to encrypt credentials locally.

C) The lack of isolation between my data and site code means that I don't have defense in depth if the credentials get compromised. If someone breaks into my session, they can't only compromise my database, they also have push rights to my entire repository. This is especially problematic for apps where I want multiple people to have access, since I often don't trust them to keep their passwords secure.

D) To expand on that, you can't do any validation of input at all -- clientside validation doesn't count. Your only permission you can grant people is "100% commit access to my entire repo, plus (potentially) the ability to execute arbitrary code during my build process", or "nothing".

E) It's also just honestly a lot more complicated than normal hosting. There ended up being a lot of niggly details that I was fiddling around with that weren't terrible to solve, but left me asking, "what exactly is my end-goal here and is it worth the time I'm putting in?"

The security issues mean that having an open submission form is a pretty bad idea anyway, so really you'd only want to do this for a personal site. Except -- I can't think of any scenario where I want to edit a personal website, I have an Internet connection, and I'm on a computer I'm comfortable typing Git credentials into, but I don't have Git installed. Usually it's just going to be easier to make the edit locally and push to Git the normal way.

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If you are trying to build a multi-user submission form:

You can build a purely static site on Netlify and use 100 hours of 'serverless' functions for free. Your functions can still do stuff like push changes to Gitlab and trigger Gitlab builds if you don't want to fiddle with a database, except now you can actually run real input validation. Plus, if you're using serverless functions, you can make your submission forms work without Javascript, which is a nice plus.

At the point where you're doing more than 100 hours of work a month running builds, you're also probably at the point where the pure-JS repo solution is going to have a really hard time scaling. And if you do reach the point where 100 hours doesn't cut it, you can scale up Netlify functions without rebuilding your entire submission form to use a different architecture.

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None of that is to say that the setup Sammi describes isn't worth playing around with, or that you can't build a working site with it, or that people should be discouraged from trying. It's very cool; and it's useful to try things like this even just from the perspective that they're fun projects that will make you a better coder.

I just question whether it's really a holy grail, or even a good idea to hook up to your own Gitlab account. I would posit that the reason you see fewer people actually following through and adopting Isomorphic Git in this way isn't because no one's thought of it, it's because you can build the same solution more efficiently and safely for the same price ($0) using serverless functions or (worse-case) setting up a single $5 Linode VPN that you share between all of your sites.