Gronk had 308 receptions in his first 5 years in the league (2010-2014), Witten had 348. Gronk had 4,379 yards while Witten had 3,983 yards. I won't even bring up TDs because it's not close. Those stats are with Gronk being injured for the better part of 2012 and 2013. Now, Gronk could continue to get inured, and that would definitely change things of course, but even if he stays relatively healthy he'll beat Witten and probably Gonzalez in most receiving stats. Witten is a top 3 all-time TE and going in the HOF. I'm just saying Gronk could be the GOAT by the time his career is done.
Now, if Witten had played with Brady at any point in his career things would probably be different