Gronk is staying retired. From all the horror stories I've read about players playing through injuries, and team doctors pushing them through insane pain, I genuinely think he doesn't have a lot of respect for anyone but the players.
He has a wealthy family that was always trying to keep him healthy and always made him consider getting second opinions on injuries. That's because NFL doctors are absolute garbage and will push you through anything they can. At some point, if you're given the other perspective so often, you start to realize - you are a commodity, and when your body craps out, you're taken to the trash. I think he finally had enough. Go him. I will legitimately be sad if he suits up again, his body has taken too much of a toll. What the hell is there to enjoy in life if you are enduring constant, constant pain.