Gronk's body language suggested that (a) he didn't think he was interfered with and/or (b) that he knew he couldn't catch it anyway. It's not until the flag comes in late that he perks up a little and starts clapping. Seriously, just about every WR and TE in the league would be making the flag gesture if they were breathed on wrong while running their route, especially on a game-winning attempt. Gronk didn't even do that, he just let his shoulders slump and accepted the loss until the penalty was called.
There's no way in hell he could have caught it anyway. His momentum was going to the goal post and his hips were turned the wrong way. Short of breaking the universal laws of physics and assuming the DB who caught it was nowhere in coverage, it still wouldn't have been complete.