You create a set of conventions that “Gemini compatible” websites conform to. This will allow your site to work with small Gemini-only browsers. Those conventions mostly amount to sending markdown for URLs when the user agent sends “accept: application/x-Gemini-markdown” (or whatever the mime type would be). Web servers send the markdown to Gemini user agents while they send normal html to browsers that don’t send the Accept header.

To signify that the site is a “Gemini site” at the URL level you can create a convention that the url must end in “.gemini” e.g. https://foo.com/blog.gemini

> (Almost nobody would vary the response based on the Accept header. Besides, if they did, you might as well just set an X-No-Ads-Please header and send HTML in both versions)

I don’t see how sending a different file based on an accept header is any more technically unrealistic than creating an entirely new technology stack.