GraphicsMagick пишут, что они гораздо лучше, и откололись от ImageMagick в 2002 году, когда те перешли на лицензию, ограничивающую использование их кода.

GraphicsMagick is much smaller than ImageMagick and has dramatically fewer dependencies on external libraries. For example, on the FreeBSD operating system, a fully-featured install of GraphicsMagick depends on 36 libraries whereas ImageMagick requires 64. GraphicsMagick's installation footprint is 3-5X smaller than ImageMagick.

GraphicsMagick is usually faster than ImageMagick. The baseline execution overhead for simple commands is much lower, and GraphicsMagick is also more efficient at dealing with large images.
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GraphicsMagick project was started in November 2002, ImageMagick from ImageMagick Studio LLC abandoned the MIT X11 style license it had been using since 1992, and switched between several different licenses until it ended up with one based on the Apache license, which is intended to penalize projects which borrow some of its source code, or fork from it.

http://www.graphicsmagick.org/FAQ.html#how-does-graphicsmagick-differ-from-imagemagick