I think BB compared him to Kyle Brady when he was drafted. I'd say Gronk is a better athlete, but Brady was known as almost a 3rd tackle when he played for the Jags.
The TE position is really loaded in the NFL right now. It's almost like every team has a good one. If your looking for pure pass catching then it's Gates and there's a big gap between him and everyone else, he's a better pass catcher than 90% of the WRs in the league. For overall blocking and catching, then I'd say it's between Vernon Davies and Witten.
Gronk will do well to be as good as those 2, but he looks to be a solid TE that has consistent hands, something we've needed for years.