I have not tested this on GraphicsMagick, but ImageMagick and GraphicsMagick's command line interfaces seem identical. So miniMagick should work fine with GraphicsMagick. If you get it to work leave us a message.
Does mini-magick support Graphics Magick at all? Is it your plan? From what I have heard Graphics Magick is more stable... though I have no first-hand experience with either just yet. Thanks!