GraphicsMagick in Drupal 7
Many years later, but for future reference, the problem might be related to the fact that you are using the PECL gmagick PHP extension. I believe the ImageMagick module is not meant to work with the gmagick PHP extension. There is a module meant to be used specifically with PECL's gmagick: https://www.drupal.org/project/gmagick . I hope that helps somebody.