Gronkowski might be the best blocking TE in the league. Hernandez is actually the better receiver (which is no slight to Gronkowski), because he's actually just an over-sized WR.
Gronkowski is like an underweight O-lineman that can run and catch. He's a matchup nightmare because he's massive AND an excellent receiver. Corners could cover him, but would never be able to tackle him. Hernandez is actually able to beat corners as good as Cro when they're playing man.