It's a wash at this point.
Gronk a better blocker.
Gronk had better consistent hands.
Kelce more athletic
Kelce better speed
Kelce runs better routes
Both are gamers in the red zone with Gronk having the slight edge only because TB was throwing to him and Kelce had Smith BUT things have changed in KC
Maybe when it is all said and done for Kelce, PM2 will have vaulted Kelce above Gronk and make it into the HOF with better stats.