The new Windows build page http://valokuva.org/magick/ contains builds for both Imagick and Gmagick. All builds are done using VC9 and PHP 5.3. ImageMagick builds are QuantumDepth 16 (default). GraphicsMagick builds are QuantumDepth 8 (default).

You can find these latest builds on the Windows PHP.net site along with the latest PHP 5.3 builds.

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. [...] 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.

You can grab the extension from its PECL page.