Guess it must be registered with your wayland compositor first. Not sure about a standard wayland compositor used by Gnome (or if that’s up for the user to decide), but perhaps the compositor needs a file “00-keyboard.conf” somewhere, where user-settings (perhaps set in Gnome GUI) override system-wide settings (as usual).
From a shell, you can simply do
sudo loadkeys fr or something alike. I always have to do sudo loadkeys de-latin1 in my case.
localectl list-keymaps | grep fr might help you finding the correct one.
Thanks Uwe, very helpful links, worth keeping in my notes.
It turns out mine is just plain fr keyboard, but it is nice to have so much info on it, plus the Nautilus notes will come in handy if they fix the file manager.