GraphicsMagick-devel contains the Libraries and header files you'll
need to develop GraphicsMagick applications. GraphicsMagick is an image
If you want to create applications that will use GraphicsMagick code or
APIs, you need to install GraphicsMagick-devel as well as GraphicsMagick.
You do not need to install it if you just want to use GraphicsMagick,
See also: GraphicsMagick, GraphicsMagick-c++-devel.