how to draw text in gosu

draw_gosu_text.txt

require 'gosu'

class GosuWindow < Gosu::Window
  def initialize
    super 640,  480,  false
    self.caption = "gosu"

    @font = Gosu::Font.new(self, Gosu::default_font_name, 20)
  end

  def draw
    @font.draw("Hello World!", 0, 0, 1, 1.0, 1.0, Gosu::Color::WHITE)
  end
end

window = GosuWindow.new
window.show