I spent some time thinking about how to do word-wrap for non-fixed-width fonts, and came up with this. It's pretty simple, but works well and seems fast enough for me. I thought someone else my like it, too!

Here's the wordwrap method:
  def wordwrap(message,width,font)
    word_array = message.split(' ')
    lines = [word_array.shift]
    word_array.each do |word|
      if font.text_width("#{lines[-1]} #{word}") < width
        lines[-1] << ' ' << word
      else
        lines.push(word)
      end
    end
    return lines
  end

and here's a demonstration of use:
#!/usr/local/bin/ruby
require 'gosu'

class MyWindow < Gosu::Window
  def initialize
    super(640,480,false)
    @font = Gosu::Font.new(self,Gosu::default_font_name,20)
    @message = "Hello and welcome to my very long text string which is also pretty boring and stupid, but oh well I guess there is a time and a place for lorum ipsum, and this was probably it, but I didn't have any handy so we get this crap instead, which is too bad really, because Lorem Ipsum is kinda nice, but oh well, what can you do, other than end this damn string already?"
    @margin = 20
    body_width = self.width - 2*@margin
    @lines = wordwrap(@message,body_width,@font)
  end
  def wordwrap(message,width,font)
    word_array = message.split(' ')
    lines = [word_array.shift]
    word_array.each do |word|
      if font.text_width("#{lines[-1]} #{word}") < width
        lines[-1] << ' ' << word
      else
        lines.push(word)
      end
    end
    return lines
  end
  def draw
    @lines.each_with_index do |line,index|
      height = @margin + (index * @font.height)
      @font.draw(line,@margin,height,0)
    end
  end
end

Of course, by now I've discovered that this feature is already available as part of the 7 argument form of Gosu::Image.from_text() . Unfortunately, Gosu::Image.from_text doesn't work. I've filed an issue: issue 60

But your version is useful for those cases where you can't just interrupt the game's flow with the creation of an Image, something that might be costly (it currently is so on the iPhone and I'm not sure it's easy to fix) :)

I've seen the bug and hope it was really the problem that I put into the comment. That part of SWIG is really tricky and it took me a while to figure it out both times it broke :(