GraphicsMagick is a replacement for ImageMagick. Now sci-geosciences/openstreetmap-icons depends only on media-gfx/imagemagick.
Sorry about the mass reassignment, but this undertaking really shouldn't burden bug-wranglers further.
Did you really test the building with graphicsmagick and no imagemagick around?
No, I didn't. Feel free to resolve as INVALID, if it is so.