Chances that a 31 year old Gronkowski is GRONK!! again are not great. His style of play led to injury in his prime years, so the chances of his holding up to a 16 game grind are not 100%. Then again, so are the chances of a 16 game season.
He has one year left on his deal, and no guarantee to play beyond that. So a 4th rounder for him plus a 7th is a fair deal. The Pats got a 4th for a player who wasn't going to play for them.