I disagree. Gronkowski improved the offense the most when Hernandez was not there. After Hernandez left, the Patriots were over 1 yard per play better in the run game, and significantly more than 1 yard per play in the passing game. Those were his best seasons statistically, but it depends on which statistics we value most, personal or team. Neither answer is wrong.
Another point you made was that BOTH Gronk and Hernandez played in-line TE at times. That was a point I made as well about Gesicki and Pitts potentially working together. Which one is the blocker? Neither is good at it, and while Pitts is better at it, every DC out there would want you using Pitts as a blocker--much like they always wanted Dan Marino to hand the ball off. It defeats his greatest strength by choosing not to use it. Clearly Gesicki is in the same boat ... loses much of his value if he is blocking. Neither of these guys is the blocker Gronk is .... not even close.
There are traditional roles in NFL offenses, and no matter who fulfills those roles the roles exist. For example, the offense wants 1 guy with elite speed to keep one safety high--opening up the middle for routes. Typically they want an audible option to keep in pass protectors to deal with blitzes, and these usually include 1 RB, 1 TE, and/or both. The heyday of the Buddy Ryan (and Rex Ryan) overload blitzes--which are still very popular today--is when the offense locks itself into not having enough blockers. The entire benefit of the Wildcat formation was that it provided one extra blocker. So numbers matter, and those blockers need a level of competence.
Defenses today gamble a lot. They pull lots of stunts, area overloads, CB and SS blitzes, zone drops, lots of gambling because offenses are scoring a lot of points with this new ruleset ... so to get a stop, defenses are gambling. Without the flexibility on offense to audible into more protection ... a team is going to be dominated by a defense. To a certain degree, the NFL chess game is very complex. But, there is a basic tic-tac-toe level of it that deals with numbers. Without those audible blockers, the Defense will overload blitz you and sack your QB a LOT, because without those blockers available--even if they do not block on that play--then you've just given the DC the first two moves in a tic-tac-toe game ... and he's going to beat you.
So the reality is that you still need 1 WR with elite speed ... we'll say that's Will Fuller/Albert Wilson/Jakeem Grant/Kenny Stills or whatever player you want to plug into that role. It is definitely not a TE. You'll need 5 OL and 1 QB. That's 7 of the 11 positions. The remaining 4 need to fill these roles: 1) RB, 2) TE, 3) Slot, 4) WR. That WR is is generally your primary WR#1 who generates most of the targets and receptions. Currently that's our DeVante Parker role, so that's the role I'm saying Pitts would need to play. The other receiving position is the Slot, and that's the position that both Gesicki and Pitts both play. In today's NFL it's called the Move TE, but it's a variation of the Slot. A Move TE requires the RB to be good at blocking. A two TE set requires one to be the Slot (Move TE) and another to be the TE (in-line TE). When you play a Move TE and a Slot ... that's essentially a 4 WR set. If you play that too often, defenses will destroy your QB and you cannot stop them.
Pitts can be great as a Move TE. Pitts can be good as a WR#1. Gesicki can be great as a Move TE. Any other role for him is considerably less effective. IMHO, we should pay Gesicki and ADD a true WR#1 who can be great ... like Ja'Marr Chase. That's how I see it.