I can agree on a holding call, but it wasn't catchable so there should have been no PI. We can argue till the cows come home about the contact that was made, but I observed that Gronk only turned to look back and did not try to come back to the ball. If one is trying to catch the ball and/or knows they're getting that type of contact, why would one not try to go back and catch it and at least draw the penalty? He realized that he was 3-4 yards past where the ball was thrown and didn't try to come back to the ball. It lead the referees to believe it was uncatchable.
I have no horse in this race, btw. If anything, I think Carolina winning helps us (with tiebreakers, of all things) and could potentially keep the 9ers out of the playoffs so I don't care about them picking it up. I wouldn't have minded seeing another huck into the endzone.