Timeline and twtxt-php, don’t support Gemini, only HTTP/S, as a design choice (although originally it was intended to work on Gemtext, it was a niche inside a niche, so it was discarded very soon).

At the moment of building the engine there weren’t many Gemini URLs supporting twtxt 1.1 (with twtxt.dev extensions).
Also User-Agent won’t work there, and many Gemini URLs are a mirror of the HTTP one, so I think is not strictly necessary.

my 2c