There are two circumstances in which trading Gronk would benefit the Patriots. Both are situations in which he doesn't end up playing for another team due to either retirement or injury. So regardless of how great of a player or pick we would get for Gronk, it isn't a true win for the Patriots if Gronk is playing for another team.
Having seen Bill make trades, he doesn't seem to trade offensive weapons who will be ultra successful outside of the Patriots' system. If you look back to this past season and the theories of why Jimmy G. went to San Francisco, you can see what I mean.
If Gronk were to go to another team, they would likely need to go to him much more often than the Patriots. Going to him play after play would increase the opportunity of him getting injured and being asked (forced) to play when he might need some time off.
In order to stay healthy this season Gronk succumbed to the TB12 method and seeing Alex Guerrero. If you've listened to sports radio in Boston then you've certainly heard that there's some sort of tiff between Belichick and Guerrero and that's why Brady and Gronk are unhappy. Regardless, Guerrero is based in Foxboro and is in business with Brady. If Gronk is that bought into the guru, then he better stay put. Now, should Brady retire before Gronk, then he can probably get Guerrero to follow him.