The numbers don't do Gronk justice, largely in part to injuries. I don't think any tight end in his time has been as dominant a force both as a pass catcher and blocker. Jimmy Graham and Gates have been solid pass catchers, but they do not block (especially Graham).
A couple quick stats: Gronk has more touchdowns than Witten and Shannon Sharpe and he's played for fewer years than those guys. He's averaging 15.1 yards per reception, which is comparable to Randy Moss and Julio Jones. When you consider how many redzone targets he gets, that only makes his YPC number even more impressive.