The issue with Planet Computer's Linux support is the lack of it. They all rely on custom kernels using Android drivers and libhybris to function. For both the Cosmo Communicator and Gemini PDA they glue together a bootable version of Debian, tick the checkbox for "it runs Linux" and then call it a day.
Well, there's a reason I don't recommend them; my Gemini PDA, rooted on the Android side, was a nicely serviceable little writer's tool and portable terminal, until a poor battery protection implementation bricked it.
https://www.oesf.org/forum/index.php more of a historical collection of stories than an active forum about Planet Computer's devices.