I recently got a Gemini PDA (WiFi). To run rooted Android and Linux (dual boot) I did the following:

1. Followed the details here.
2. Downloaded & unzipped Windows Flashing Tool
3. Downloaded & installed Windows Drivers
4. Chose my Partition setup here then downloaded
   1. Scatter File
   2. Base Firmware (ie Android)
   3. Linux Firmware
5. Unzipped both firmwares into a dir & put scatter file in there
6. Connected Gemini powered off
7. Started flashing tool, selected scatter file (following this guide)
8. Chose “Firmware Upgrade” & “Download”
9. Powered Gemini on
10. Waited. First you get a fast red progress bar (partitioning?). Then you get two slow yellow progress bars (presumably installing each OS). In total the process took 35-45 minutes.
    Update: on a recent install (see step 11) this was much quicker, a couple of minutes.
11. Powered on Gemini into Android (my default). To get “proper” root I installed Magisk Manager from here.
    WARNING: latest Magisk Manager put my Gemini into a boot loop requiring firmware reinstall – so don’t update to latest version & allow it to rebuild!

Note I skipped backing up the NVRAM. You probably shouldn’t do that.

Then post-install I installed Linux Deploy from the play store, Arch Linux under that (into an ext4 partition on my SD card) & termux as a terminal into it.