I can tell you exactly what Gronk was doing, since there was a closeup on him during the game.
Gronk does a stutter step and beats his man deep, then kind of gives up on the play when it looks like Brady's getting sacked. But when Brady scrambles, he just keeps running deep. So if the throw was deep enough.
So yes, if the throw is deep enough, he's scoring a TD untouched. However, as it is pretty clear from what happened during the game, if the ball is underthrown, Gronk is in no physical shape to cut back. I think it was an unnecessary risk.
As for Branch, the corner is playing deep. If you look at ~0:10, you can see the safety coming, but he's late -- so if Brady isn't so focused on Gronk, he can easily make the throw (he'll have enough time, since he doesn't have to wind up for a 50-yard hurl). But he never even looks into Branch's direction. Very un-Brady-like.