Planet's update #49 is out and covers Linux on the Gemini.
The device ships with only Android installed.
Gemini are going to provide a tool that runs on a desktop PC (Windows, Mac, and Linux will all be supported) that can flash a Gemini to dual-boot Android/Linux, single boot Linux, or single boot Android. You'll be able to change configurations as often as you want.
They have been making progress on device support in various Linux distributions, and show photos of a production" Gemini PDA running Debian 9, and also Sailfish OS, Ubuntu 17.10, and Postmarket OS (a Hildon UI based distro — similar in look and feel to Maemo/MeeGo).
They add, "Please note that although many functionality are already working (including 4G connection on Sailfish!), these are still early days and as such we still have to deal with performance issues, missing drivers, keyboard layouts and other issues."