GraphicsMagick is a fork of ImageMagick, emphasizing stability of both programming API and command-line options. It was branched off ImageMagick's version 5.5.2 in 2002 after irreconcilable differences emerged in the developers' group
Here are some reasons to prefer GraphicsMagick over ImageMagick or other popular software:
GM is more efficient than ImageMagick so it gets the job done faster using fewer resources.
GM is much smaller and lighter than ImageMagick (3-5X smaller installation footprint).
GM is used to process billions of files at the world's largest photo sites (e.g. Flickr and Etsy).
GM does not conflict with other installed software.
GM suffers from fewer security issues and exploits than ImageMagick.
GM participates in Google's oss-fuzz project (since February, 2018).
GM valgrind's 100% clean (memcheck and helgrind).
GM passes rigorous memory error testing using ASan.
GM passes undefined behavior testing using UBSan.
GM comes with a comprehensive manual page.
GM provides API and ABI stability and managed releases that you can count on (ImageMagick does not).
GM provides detailed yet comprehensible ChangeLog and NEWS files (ImageMagick does not).
GM is available for free, and may be used to support both open and proprietary applications.
GM is distributed under an X11-style license (MIT License), approved by the Open Source Initiative, recommended for use by the OSSCC, and compatible with the GNU GPL.
GM source code is managed in Mercurial, a distributed source control management tool which supports management of local changes. The repository history goes back to 1998.
GM has 0.00 (zero) defects per 1000 lines of code (293,341 total lines included) according to Coverity analysis on May 25, 2015.
GM developers contribute to other free projects for the public good.
I started with ImageMagick, I now precede any of those commands with gm