Check out the app
FinQwertyKeyboard Layouts by Anssi Hannula (
https://play.google.com/store/apps/details?id=fi.onse.qwerty.finnish).
It has a specific keymap for the Gemini, which is identical with the original one for swedish/finnish, except for swapping the "ä" and "ö" characters. However, the original method of modifying the keys with ApkEditor is working just fine for all the other characters. With "working just fine"" I mean, that it can be successfully uninstalled, altered to your own likings and reinstalled.
As the layout is for swedish/finnish devices, obviously you have to change some more mappings in order to make it usable, in case you are using a different physical layout. You should be able to simply copy your changes over from the .kcm -file that you tried to modify on the original keybord apk, however.
I have successfully used the app/keyboard map since I upgraded my Gemini to Android 8 – and I am also currently typing this reply on my Cosmo using it. Even the shortcuts for the keyboard-light (Fn-Shift-b and Fn-Shift-N) work as expected, as they are handled on a lower level.
(I use it to get access to em-dashes, "~" etc). The only key-combinations that are not possible to modify are those that are lower-level (such as "play", "forward" etc).