ESPN's Jeff Darlington reports Gronk wants to continue to play with Tom Brady but mentions the tension with Belichick has been ongoing. Darlington does note that things can be fixed between the two.
There have been hints of frustration on Gronk's part all throughout the offseason. Whether it was telling Danny Amendola to "be free," mysteriously tweeting motivational quotes or subtweeting reporters, there really is no shortage of clues that insinuate Gronk isn't happy right now.
On top of all of this, there are trade rumors surrounding Gronk. No matter how much a player denies it, trade rumors don't sit well with anyone. It may wind up being nothing, but this has to tick Gronk off at least a little bit.
The Patriots have won without Gronk before, but life without him would be difficult for everyone from fans to players to get used to.