Hi, I see that asciidoc is depending on ImageMagick directly. The GraphicsMagick program is (usually) a drop-in replacement for ImageMagick. To support them both, we have virtual/imagemagick-tools. Since asciidoc doesn't link against either one (it just runs "convert" in music2png.py, as far as I can tell), all you should have to do is replace "media-gfx/imagemagick" with "virtual/imagemagick-tools[png]".
I've added the USE=png condition since I suspect music2png.py won't work without PNG support.