There is several Gemini sever software packages to pick from. While I was looking I came across two videos.
How to install and set-up a gemini server
How to make a Gemini Capsule - The protocol, not the rocket. So you too can be cool! (with uoou)
Both videos use the Agate server, so that is what I decided to try. The link to the Agate server is below.
https://github.com/mbrubeck/agate
Chris Were Digital, the author of the first video, has a set of notes you can follow. Which is what I did. The link to the notes is below.
gemini://chriswere.uk/gemserver.gmi
Just follow Chris's notes and you will be up and running in no time.