[donio]

For Emacs users elpher.el is a good option. There are also some additional packages to help with authoring .gmi files either locally or over Titan.

https://thelambdalab.xyz/elpher/

https://git.carcosa.net/jmcbray/gemini.el

https://alexschroeder.ch/cgit/gemini-write/tree/README.md

You can find these in MELPA too.