Bully Dog Tuners for 2004 2005 Chevy GMC Duramax LLY Diesel Trucks. Bully Dog's GT tuners are incredibly popular with the diesel enthusiast community. They are a remarkable four-in-one performance product that provide: a vehicle engine tuner, monitor, gauge display, and diagnostic device all in a single unit. The 6.6L Duramax engine, in a Sierra or Silverado, responds incredibly well to the GT Platinum tuner. If you want more out of your truck, whether it's for sport, competition or towing - you can't go wrong with a Bully Dog at the helm.