I think it was more the return the Panthers got back for Guddy that POed so many of their veterans. Getting questionable futures (certainly not helping them now) in return for a big minute only hurt their team, as proven by their dismal season keeping pucks out of their net. Was it not Gallant who openly said losing Guddy was the biggest reason behind their poor results? "Gerard Gallant would never admit it (at least not before Sunday), but he hated seeing Erik Gudbranson traded [...]Gallant valued Gudbranson’s game, how hard he could make it on opposing forwards."
From this article it appears Gallant was fired (soon after signing a long extension) over his anger about the Gudbranson trade.
There is a lot more to evalutating a player's effect on a game than analytics. They are a means to an end, but not the end itself.
http://www.sportsnet.ca/hockey/nhl/different-visions-led-split-gallant-panthers/