Haven't used my Gemini in some time.....not because of lack of interest, but other reasons....
Think there are some tweaks you can do....suggest looking through various prior posts....
As for other linux distros......
You would need to find one that has been compiled for ARM CPU's....
Unfortunately, LinuxMint does not support ARM.
Haven't tried it, but there is version of Ubuntu for ARM.
Think issue you may run into is while it may run, it may not display correctly for the Gemini's display...ie, may need some tweaking, I would guess.
It's clear that Planet is limited on resources, from what i see their strategy is to leverage existing OS's like Android and Sailfish, that need minimal tweaking, reducing their need for expense on their part.
Linux/Debian, from what I see is clearly an afterthought, relying on efforts by others outside of Planet to support it. And thinking back to some early Planet videos about the Gemini, it's clear that was their plan, if you listen carefully to what they said at the time. They really didn't commit themselves to supporting Debian directly. Rather they were saying that the Gemini would be able to run Debian, but really said they'd develop it themselves. At least that's my recollection.
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Meanwhile, having said this, Adam Boardman, has done a very good job on his own, I think, in making Debian available, and doing some tweaks to it. After reading various posts here and playing around with it for awhile, I was able to get Debian running OK.
So it goes....
Mark