Well, for the time being, I've decided to forget about rooting the Gemini, much though I'd like to have done it. Likewise for adding other OS's. I didn't like Sailfish. Much too flashy for my taste. I've tried Debian, before, on desktops and on the Pi, and didn't like that either. As an ex Sys Admin on UNIX, a properly installed Super User is an absolute must. I run Mageia on my desktop, as being the closest I can get to a proper system, and in that, I'm not happy with updates only requiring a user password. Also, marrying a standard Debian install, meant to run on a large monitor, onto the tiny screen of the Gemini, as good as it is, is a definite nono.
I really have given it a good try. Without exaggeration, I must have flashed my Gemini fifteen to twenty times with all sorts of combinations. Even to the extent of clearing all and every file from the loading PC and starting entirely from scratch. Apart from the very first installation, and that had it's own hangups and problems, not one of the installs has resulted in a successful conclusion. The actual flashing has always gone without problems, but it's the installation of Magisk Manager is where it all falls apart. Apart from the very first and finally successful installation the indicated popup with the message ""Requires Additional Setup" has never happened. Even in the only successful install. the popup did not appear for some time, perhaps half an hour, after Magisk Manager had been installed. That the rooting had in fact taken place, as I've indicated, by installing and opening the Barclay's Online app, worked every time I used it. Barclay's programmers, obviously knew how to detect a rooting operation, why is it seemingly beyond the scope of anyone else.?
I did try contacting Gemini Support, who got back to me commendably quickly, but my question as to how one progressed beyond the point where an expected popup did not popup, went largely unanswered, and information as to how to move from that point was totally absent.
So, I'm at present stuck with Googles multiplicity of unwanted apps, in an OS which has all the appeal, logic and nicety of an early DOS system.