This allows the standard Monitor Brightness Up and Monitor Brightness Down keys on my Apple Keyboard to control the backlight brightness on my 27" Apple Cinema Display when running Ubuntu 20.04.
Gnome doesn't allow directly rebinding the Monitor Brightness Up or Monitor Brightness Down keys.
Instead one must alter their actions at a lower level.
ACPI is how Linux systems normally control monitor backlights (ls /sys/class/backlight/), but there is no such driver for Apple Cinema Displays.
Fortunately there's a simple command line tool acdcontrol which we can trigger in response to ACPI events.
Pull and compile acdcontrol:
git submodule update --init
(cd acdcontrol && make)
Copy the necessary files into /etc/acpi/ (requires root):
sudo cp events/acd-* /etc/acpi/events/
sudo cp acdcontrol.sh acdcontrol/acdcontrol /etc/acpi/
Restart acpid:
sudo systemctl restart acpid
If this doesn't work for you, use acpi_listen to check that these events are firing when you press your brightness up/down keys. You may just need to tweak the event string.