I mean the format for selecting paths and so on. Git already has a standard patch format for sending changes via email and a builtin utility for applying them to a repo, so why not use that format?

But I need to have git installed. I simply want to send something to my repo via email from my smartphone.

For the mentioned workflow in chatgpt I'm guessing chatgpt can generate the email body for you in the correct format, but sure it is a step over just copy-pasting the raw text.

There is an existing, well-established patch email workflow which git natively supports. Using that, and extending it when necessary, would make more sense.

See git-send-email and git-am.

But therefore I need git installed, right? I want to use it i.e. to commit some text via mail on my smartphone

No, but you need to be able to write a well-formatted diff, which is pretty much the minimum requirement for sharing changes.

Emailing the entire new file every time will not work in anything but local tests as it cannot handle change conflicts. The format needs to know which parts of the file you changed, so your change does not accidentally undo other changes merged ahead of you.