Although the available information is spread very thinly, it still exists. These are resources that might be useful for those in search of more, including those I draw my knowledge from.
The official support wiki
Planet Computers provides a small wiki with a few support pages at support.planetcom.co.uk
It mostly focuses on where to download images with GNU/Linux distributions as multiboot options in them, and how to flash them to the device.
Gemini Planet is a website run by fans of the Gemini PDA and the Cosmo Communicator I think emerged from a fanpage on Facebook. It’s not an official place for support, but harbours some useful information on those devices.
The Gemian (Gemini-Keyboard-Apps) wiki
This is a github hosted wiki attached to a Github project supposedly focused on “supporting maximum keyboard navigation options for use on the Gemini”.
Looking at the right places in the wiki, some insight on system specifics can be found. The persons writing it have notable insight on the Debian image for the Gemini and how it is built.
There is a subreddit dedicated to this clamshell device: r/geminipda