How was the ball uncatchable when the initial second contact was an arms length from where it was eventually caught? The first, relatively minor contact, was by the guy who caught the ball, from behind the receiver.
It was uncatchable because Gronk was moving himself away from the ball. If he’d have tried to turn back towards the line of scrimmage (coming back to the ball) he’s have been interfered with, but he continue moving towards the end line on his own the whole time. The fact that he was moving perpendicular to the guy that caught the ball shows it was uncatchable, he needed to move in a different direction than he was.