After unlocking the Grim Ripper, select him as your guitarist in Career mode, then exit Career mode. Next, go to Quick Play mode, choose any song at any difficulty, and either finish the song, or quit once it has started. Then, go back to Career mode, and the Grim Ripper will be playing the Gibson Les Paul with the Cherry Sunburst Classic skin! To make him play the scythe again, simply select it in the "Change Guitar" menu.