Poor maximum JPEG quality.
The maximum JPEG quality is poor comparing to other tools, like ImageMagick or Photoshop.
The colors are lost even at quality=100.
See http://oserv.org/bugs/gd/ for original (.png) / GD(.gd.jpg) / ImageMagick (*.im.jpg) comparison.
Everything was done using maximum quality settings.
Was tested using php and perl, the results are the same.