Prior to the 2013 season, Gronk received the largest tight end contract in the history of football, just after finishing his third season in the league. It locked up Gronk with the Patriots through the year 2019 and ensured that he'd be with Tom Brady for the remainder of Brady's career.
The wording of contract was a six year extension, on top of the remaining two year of Gronk's rookie contract. The value was effectively the same as finishing his rookie contract and then getting franchised for the next two season. The remaining four seasons are the result of a team-option that could keep him in the fold for an additional four seasons, and that's where Gronk makes most of his money.
This option could remain a point of interest, should Gronk be unable to figure out his health issues. While it's extremely unlikely the Patriots will cut Gronk in either of the next two seasons (his dead money would cripple the free agency periods), it's possible they could just walk away from Gronk after the 2015 season.
In all honestly, it's crazy to think of the team letting the most impressive tight end in football history just walk away, but it's something the Patriots need to keep in play if Gronk continues to be unavailable during the season. Additionally, it makes drafting a tight end with a similar skill set to Gronk even more viable, since they would be able to move forward with a viable contingency plan after two more seasons.