Quoting official README,
The Duckling proxy is a scheme-specific filtering proxy for Gemini clients
to access the web. It behaves as a normal Gemini server, except it
retrieves its content from the web.
You can tailor its behaviour when it starts, to tailor how web pages are
transformed to gemtext.
It is scheme-specific, i.e. it is designed to handle HTTP requests only.
Web pages are translated to text/gemini. Other web resources are returned
directly.
The primary intended use case for this proxy is as a personal proxy to make
the web accessible to your favourite Gemini client.
Dmitry Bogatov