Antonio Gates & Tony Gonzalez were more superior pass catchers than Gronk.
Travis Kelce when it’s all said & done will surpass Gronk as well and already has more career yards than him. Edit: Actually they’re pretty much even.
Gronk is a HOFer for sure, but no where near the best. I’d put him at the Jason Witten tier.
These are just names off the top of my head. I’m not old enough to have seen Kellen Winslow SR play, but I’m sure he’s up there as well.