Now the RB Mix song that you can listen to here does not have any additional instruments in it, only the Acoustic Guitar. However on the original soundtrack we get to hear more instruments. In that song it is mostly the acoustic guitar and a Trombone or a Trumpet playing along. I can't really tell if it's a Trumpet or Trombone, could be a pocket trumpet for all I know. There is an electric guitar that does a short but great solo at the end and it livens up the song.
I also did a Gungrave wallpaper for you. You can check it out below. It's 1280x800 and the picture is from the first episode of the Gungrave anime called Destroyer in The Dusk. I think it turned out pretty good, so enjoy the music and the custom wallpaper.