Another way to do this is through the
dconf-editor. This method has a few extra steps from
gnome-tweak-tool, but is useful if you don't want to pull in the dependencies from the tweak tool.
This will allow you to use the
caps:swapescape syntax and automatically make the change permanent.
sudo apt-get install dconf-tools
After starting the
dconf-editor, navigate to org >> gnome >> desktop >> input-sources
Add the options that you need in
xkb-options. The option strings are surrounded by single quotes and separated by commas. Be careful not to delete the brackets on the ends.
You can use this method to enter most of the traditional xkb options that are no longer available in System Settings >> Text Entry. The exception are the settings for switching the keyboard layouts, which currently do not work because of a bug.
For a list of the options and the syntax, use
man 7 xkeyboard-config in a terminal.
Another common option that could be used is
terminate:ctrl_alt_bksp to allow ctrl+alt+backspace to end the X-session.