Gnome, Cinnamon, and KDE all use xkb to manage keyboard mappings.
For Gnome, you use gnome-tweak-tool to access all of the xkb options.
I'm working on Ubuntu 21.04 using Gnome 38.8.5, so all of my xkb files are under /usr/share/X11/xkb. Consult your distro's documentation if this is not the case for you.
The first file we're going to be working with is an addition, under symbols in that xkb folder. It should already have a bunch of files like us and dvp etc. We're going to be adding a new file called hyper. So on my computer, this is /usr/share/X11/xkb/symbols/hyper.