Except that other man Russell.
I do think Shaq is a good comparison for Gronk. I think Shaq is probably the most uniquely dominant player in the history of his sport, and I think that's true of Gronk too. Obviously Shaq lasted longer, but his last decade or so was severely hobbled by nagging foot injuries that slowed him down (people forget how fleet of foot Shaq was in his early years). Similarly, Gronk was a victim of his own dominance - the only way to stop him was to launch yourself bodily at his few weak points, and those points only got weaker over the year.
Gronk's the best tight end I ever saw. Admittedly, I never saw Ditka, Casper, Mackey, or Newsome, but he was unique. Gonzalez was great as both blocker and receiver, but was never as dominant at his height. Sharpe didn't block, and Gates learned to block only after he turned 30.
Witten was good, sometimes great, for a long time as both blocker and receiver but never particularly amazing; I don't think he was ever the best tight end in the NFL, let alone the best of all-time. For much of his career, Gonzalez and Gates were better and later you have Gronk and at times Graham, Kelce, Olsen, etc. Being the 2nd to 4th best tight end for the better part of two decades isn't anything to scoff at (Drew Brees did this as a QB!) but he's not the best of all-time and I'm not sure he's even in the top 5. Gronk is number 1.