Ruby Gtk3 Tutorial
Gtk (Gimp Tookit) is a library to create programs' user interface. It is licensed by LGPL that means you can develop software which can be open software, free software, comerical non-free software.
Gtk also is used by Gnome desktop tools.
For Ruby applications you should use gem to install needed library, or sometimes system packages (deb, rpm, ..) can be enough to use a ruby libraries.
sudo gem install gtk3
Offical address of gtk3 gem: http://rubygems.org/gems/gtk3. You also see dependicies of gtk3 on the link.
Under /var/lib/gems/ you can see whole installed gems on your system.
You can run Ruby on Netbeans and Rubymine. They supply auto-completion, spell checking, coding style, highlighted text etc. If you import gems on the ides, you should specify full gem path in settings.
I strongly recommend to use pry, it is quite good shell. It has auto-completion, highlighted text.
For mor information see: http://pryrepl.org/