Gallagher's positioning and puck battling ability is exceptionally strong. Getting into passing lanes, forcing turnovers, aggressive forecheck (aggressive insofar as positioning--not physicality) and generally just reading the play and seeing the ice--this is what has made Gallagher a very strong player. He's better known for taking abuse in front of the net, but his ability to see the ice and track the puck is second only to Corey Perry on the team. Those attributes are borderline elite from Gallagher. That's also what makes him so dangerous around the net; his superior ability to track the puck allows him to get to loose pucks faster than his opponents.
I don't expect this primary skill of Gallagher's to deteriorate with age. He's also never been fast. So I have some hope that he'll maintain a high level of play. That said, it's hard to disagree that $6.5m is an obscenely high price tag for a player whose body was already breaking down by the time he became a pro.
I think the disdain for Gallagher's play in these playoffs are wholly unwarranted. It seemed to me as though Gallagher was the superior defensive player on his unit; that he clogged up the neutral zone exceptionally well; led the team in its forcing turnovers and making breakouts for opposing D very difficult all playoffs long (up until the TB series); and Gallagher is often the player on his unit that leads the zone exit (he's very, very good at lobbing the puck up ice without triggering icing, but good enough to get a line change in). He's also very dependable to get the puck deep, keep plays simple, not turnover in the neutral zone, etc. Just a very, very smart, fundamentally strong player who sees the ice exceptionally well, and can hopefully help cement these fundamentals into players like Caufield going forward.