GraphicsMagick is a replacement for ImageMagick. Now www-apps/coppermine depends only on media-gfx/imagemagick.
Sorry about the mass reassignment, but this undertaking really shouldn't burden bug-wranglers further.
This bug really belongs to the web-apps herd. Either way, I solved it by removing the dep. The package can function fine with dev-lang/php[gd], which should be available everywhere(tm).