I am excited to announce the release of the Gmagick PHP extension, which I ported from Imagick along with Mikko Koppanen, one of the Imagick author who got me to do the extension and guided me along the way.The extension seeks to make the image processing capability of GraphicsMagick accessible from PHP. GraphicsMagick is forked from version 5.5.2 of ImageMagick and had since developed with important differences and improvements (for more details, see GraphicsMagick's website).
In terms of Gmagick and Imagick, we try to keep the method interface similar so existing Imagick users will find converting to Gmagick a breeze. One major difference however, is that most of the mutator methods in Gmagick are chainable.
We'll leave you to decide which extension is more suitable for your needs. A high level difference that we keep in mind when developing Gmagick is that Imagick's feature set is more extensive and powerful, whereas Gmagick should be simpler and more efficient (see benchmarks).