I'd say slightly better than a 50% chance the Pats offer Gronk a 1-yr extension that lowers his 2019 cap hit in exchange for some guaranteed money in signing bonus that can be amortized over two years by the Pats. A significant percentage of his potential max earnings would be in the form of weekly roster bonus and incentive bonus. In fact, the Pats may insist on that rather than merely paying him to play his 2019 contract out.
IIRC, Gronk has stated publicly that he won't play for another team. If he's sticking to that, then a trade is off the table.
So, Gronk's only alternative to negotiating an extension might be to retire.