Well, damn.
I made sure my Gemini was charged, after having been off for months and months. (In all that time, it was still at 84 percent.) Did the FlashTool thing, or tried to. In the Readback screen I dutifully plugged it in and turned it on; to my surprise, the Linux login screen instantly appeared and told me that it was a date in November 2016 (though the clock was right). Anyway, the progress bar turned red and filled up but never got to the okay screen. I attempted to retry it, but the Esc key turned the device off and now it appears to be stone dead.
I used to know the tricks, but forgot them, and now I have no idea how to get the Gemini back so as to flash it with just Linux. The battery was at 100 percent, so it's not that. Is there a force on or force reboot? (Sitting on the Esc key doesn't do the trick, sadly.)
Have I bricked this thing?
UPDATE: I haven't bricked it, but am lost in the woods (I hadn't booted Linux since installing Sailfish; now, for some reason, it boots directly into Linux). Stay tuned for many questions! And thanks!