It sounds like there's a font problem -- either the font you're choosing may not have the chars you're typing, or a problem happened during a font load.
Can you share some of your code?
Update : I think -- I don't have time to test this -- that you should have, but are missing one of the following lines.

  $foo->set_font('font.ttf', 12) or die $gd_text->error;
  $foo->set_font(gdTinyFont);
  $foo->set_font(GD::Font::Tiny);
[download]