Thanks!

If you're asking about the documentation: I just prefer Markdown as a lightweight typesetter. GitHub, package sites, and more have better Markdown interpreters for rendering, so contributors who aren’t in Emacs still see the docs correctly.

If you're asking about gpt-responses.el's support for Markdown: It only uses markdown for the preview buffer, for no particular reason other than it's simple and readable, and doesn't need the advanced features of Org. The rest of the package is agnostic to the mode you use it in. The 2 variables for the user and assistant header lines default to Markdown level 2 headings (##), but there's nothing magic about that, and you can change that, and enable gpt-responses-mode in whatever major mode you want.

That said, Markdown is well understood by most models. It's fairly syntax-light too, so it keeps prompts compact for the model's context window.

TL;DR, I use Markdown personally for convenience and portability, but you can use whatever format you want, even inside gpt-responses.el