Great job! I actually started reading the source code.

In src/gemini.rs you wrote:

    pub struct GeminiResonse {
    ...

You meant ''GeminiResponse''?

I think instead of a mutable GeminiRespone, add that not_found() (and subsequently new response types) into GeminiResponse impl and construct one when needed. And then, for consistency, GeminiResponse::new() should become GeminiReponse::success().