I don't know if I really agree with this approach of presuming that DRK needs another GCD-based attack simply because it has fewer of them than other tanks, and then working backwards from there to figure out if and how you can actually do something interesting, fun, or productive with it. I don't really see what problem is being solved here, or what meaningful extension to the class's gameplay is to be gained by figuring out how you could implement Bloodspiller 2 or spin Delirium's GCD-based gameplay as being substantially different from Gnashing Fang's GCD-based gameplay.