GraphicsMagick in Drupal 7 D7 has ImageAPI and that ImageMagick Module is also installed. However I want to use GraphicsMagick with D7 but haven't been successful till now. I have both ImageMagick and GraphicsMagick installed in my Ubuntu Box, I also have pecl imagick and pecl gmagick installed. I know both pecls doesn't work at the same time, but I have been changing configs whenever necessary. 

I have tested both ImageMagick and GraphicsMagick from command line, both works perfectly, but I couldn't make it work with Drupal. According to ImageMagick Module I just need to change in Image Toolkit from convert to gm, Drupal reads the executable binary perfectly. I try to upload photo which doesn't show after processing and is also not uploaded in the file. But when I change it back to convert and pecl imagick everything works smoothly. 

Any suggestions how to use GraphicsMagick with Drupal 7 ?

I even tried that this http://drupal.org/sandbox/W32/1077458 sandbox project, but no luck...
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.