GraphicsMagick is a replacement for ImageMagick. Now dev-ruby/gruff depends only on media-gfx/imagemagick.
This is a false positive. dev-ruby/gruff depends on rmagick, which in turn can only use imagemagick. The explicit dependency on imagemagick in the ebuild is to ensure that imagemagick is built with the truetype USE flag.
Thanks for filing this, thought, because otherwise I might not have found out that this extra dependency got dropped by accident in the last version.