ruby/gem-config: add prawn-gmagick

prawn-gmagick is a Prawn extension that uses GraphicsMagick to embed image
formats Prawn's built-in code does not understand (notably WebP). The gem
ships a C extension that links against libGraphicsMagick via pkg-config, so
graphicsmagick and pkg-config must be in scope at build time.

pkgs/development/ruby-modules/gem-config/default.nix

@@ -892,6 +892,11 @@ in
     buildFlags = [ "--without-imlib2-config" ];
   };
 
+  prawn-gmagick = attrs: {
+    buildInputs = [ graphicsmagick ];
+    nativeBuildInputs = [ pkg-config ];
+  };
+
   psych = attrs: {
     buildInputs = [ libyaml ];
   };