I went ahead and tried it, I guess things went as expected. Process:
copy Gemini_Dual_Boot.txt as Gemini_Dual_Boot_Rooted.txt
edit Gemini_Dual_Boot_Rooted.txt: replace "boot.img" with "patched_boot.img"
follow Flashing instructions, except that un-check all the boxes except "boot", and use Download Only option.
Flash device, boot: now it's rooted.
Can't see that I would have lost anything in the process; software and accounts remain on Android side, and Debian side seems the same also.
Are there "official" or recommended ways to root the Gemini? So far, I was recommended Magisk, but not specifically for Gemini.
From the Gemini Firmware :" Rooted Android firmware
With Gemini you have the option to easily root your device. Please bear in mind that rooting an Android device comes with associated risks, and some application might not work correctly. You can search the Internet for more information, we suggest that you proceed only if you know what you are doing.
If you know what you are doing and you want to continue, then you can use the "Gemini_Android_Rooted.txt" scatter file to flash the rooted Android firmware. Please complete the process by installing Magisk Manager"
So since this is from Planet it is for Gemini.