While I understand the complaint to a point, GNB's Gnashing Fang+Continuation burst range is 3y, which is standard melee range, and is in this matter no different from the other tanks asides from PLD with their burst periods.
DRK spamming Bloodspiller, Edge and other oGCDs during Delirium require being in the 3y melee range, same with WAR and Inner Release Fell Cleave spam.
So then what is so different about GNB's burst to warrant a greater range cushion during it over the other tanks?