The cap hit is fine (certainly not a steal though) but the term is a major issue. Gardiner is 34 when that deal is done. Do you think he's going to be the type of skater he is right now when he's playing in his 30's? Without his skating he's not an NHLer, and that doesn't even consider his back which essentially relegated him to a bad bottom pairing defensemen when he came back.
Then we have to figure out a way to use him in such a way that he can useful, and the only way he does that is by taking away prime ice time from Rielly and Barrie. The writing was on the wall once we acquired Barrie. Gardiner is without a PP role, and we won't have much ES offensive minutes to give him either. We can't trust him defensively without having him heavily sheltered, and we don't have much time or room for that on a competitive team. Especially not one on that kind of contract and at his age. Barrie is the guy they brought in to use exactly like they used prime Gardiner, except he is a RD and has a much better history of sustained success.
Finally, just in the next year alone, Dermott is going to be knocking on the door for a top 4 job, Sandin and Liljegren are going to want opportunities, and who knows what we have in other guys (ranging from Schmaltz and Harpur to Hollowell, Duszak and Lindgren). Most of those guys are not pure defensive players. They are going to get some of the ES or even PP offensive minutes that Gardiner would have received.
Dubas made the right move to not sign Gardiner to this deal, or to any deal.