require 'gtk3' app = Gtk::Application.new('org.gtk.example', :flags_none) app.signal_connect 'activate' do |application| window = Gtk::ApplicationWindow.new(application) window.set_title('Window') window.set_default_size(200, 200) label = Gtk::Label.new('Hello, Gtk3') window.add(label) window.show_all end puts app.run