game.rb

class Game

  def initialize
    @player = Player.new
    @gui    = GUI.new(self)
  end
  attr :player, :gui

  class Player
    attr_accessor :hit_points, :max_hit_points

    def health
      (max_hit_points.to_f / hit_points.to_f * 100).to_i
    end
  end

  class GUI
    def initialize game
      @game = game
    end
    attr :game

    def display_health
      puts game.player.health
    end
  end
end

g = Game.new
g.player.max_hit_points = 10
g.player.hit_points     = 12
g.gui.display_health

~output.txt

HEALTH: 83

~pry-session.rb

$ pry
2.0.0 (Object#main):0 > class Game
2.0.0 0 *
2.0.0 0 *   def initialize
2.0.0 0 *     @player = Player.new
2.0.0 0 *     @gui    = GUI.new(self)
2.0.0 0 *   end
2.0.0 0 *   attr :player, :gui
2.0.0 0 *
2.0.0 0 *   class Player
2.0.0 0 *     attr_accessor :hit_points, :max_hit_points
2.0.0 0 *
2.0.0 0 *     def health
2.0.0 0 *       (max_hit_points.to_f / hit_points.to_f * 100).to_i
2.0.0 0 *     end
2.0.0 0 *   end
2.0.0 0 *
2.0.0 0 *   class GUI
2.0.0 0 *     def initialize game
2.0.0 0 *       @game = game
2.0.0 0 *     end
2.0.0 0 *     attr :game
2.0.0 0 *
2.0.0 0 *     def display_health
2.0.0 0 *       puts "HEALTH: #{game.player.health}"
2.0.0 0 *     end
2.0.0 0 *   end
2.0.0 0 * end
=> nil
2.0.0 (Object#main):0 >
2.0.0 (Object#main):0 > g = Game.new
=> #<Game:0x007fa88a881de8
 @gui=#<Game::GUI:0x007fa88a881d48 @game=#<Game:0x007fa88a881de8 ...>>,
 @player=#<Game::Player:0x007fa88a881d98>>
2.0.0 (Object#main):0 > g.player.max_hit_points = 10
=> 10
2.0.0 (Object#main):0 > g.player.hit_points     = 12
=> 12
2.0.0 (Object#main):0 > g.gui.display_health
HEALTH: 83
=> nil