Ruby/Tk is an old Gem.
But it still works well.
You need some option to install Ruby/Tk on Ubuntu.
Install dependencies.(There are various other packages in Tk, so you can install them as needed.
tkpng and so on.)
sudo apt install tk-dev
Install Ruby/Tk gem.
gem install tk -- --with-tcltkversion=8.6 \ --with-tcl-lib=/usr/lib/x86_64-linux-gnu \ --with-tk-lib=/usr/lib/x86_64-linux-gnu \ --with-tcl-include=/usr/include/tcl8.6 \ --with-tk-include=/usr/include/tcl8.6 \ --enable-pthread
If you install a new tk package, you might need to reinstall Ruby/Tk as well.
Have a nice day.