Well to answer the question specifically, a big reason they chose to split him out wide against Sherman, is because of Sherman's arm injury. A big part of Sherman's game is getting his hands on receivers, being more physical, and bumping them off their routes. Now Gronk has 3 inches and 70 pounds on Sherman, and with his bum arm, he wasn't able to lift it and use it to full strength, so Gronk would be able to take advantage of the size differential a lot better with a less than healthy Sherman. That's why you saw, at times in the game, other people going against Gronk, because Sherman wasn't at his best and able to be as effective against the bigger receiver like he usually is.
"The one thing that stands out on tape is that no matter where he(Gronkowski) lines up, the teams that disrupted him -- either at the line of scrimmage or within five yards of his release -- had the most success. Gronk is a bad man and if he gets a free run off the line, bad things happen."