-
Create a local database for machine-wide settings in
/etc/dconf/db/local.d/00-input-sources:
[org/gnome/desktop/input-sources]
# Set the Right Alt key as the Compose key and enable it
xkb-options=['compose:ralt']
If you want to set a different key than Right Alt, replace
ralt with the name of that key as specified in the
xkeyboard-config(7)
man page, section Position of Compose key.