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

well, I assume by syntax you mean Gemtext (which I like a lot, my personal blog is built on top of it), so I think it might work for twtxt clients...

I knew of twtxt in Gemini Antenna, so at least the 2017 spec might work on that protocol. I think the main issue with extensions is that they weren't designed with many URLs and protocols in mind.

Also I have to admit that the Gemini community significantly reduced in the last few years. I don't know how worth it is to add support for Gemini now.