Before we start, let's address one key thing, Sergei Gonchar isn't going to fix the Montreal Canadiens' powerplay. Gonchar had 22 points last season, 15 of which were on the powerplay, one fewer point than Ryan Whitney, who is playing in the KHL and can barely skate due to multiple ankle injuries that have destroyed his career. Gonchar played on the first wave of the Stars' powerplay last year, which clocked in at 23rd in the NHL, converting on only 15.9% of their opportunities. At 40 years old, Gonchar is not the elite offensive talent that he once was.
The Habs' powerplay also isn't struggling due to personnel, but due to poor tactics, and a little bit of bad shooting luck. Sergei Gonchar isn't going to have the other four skaters he's on the ice with dump the puck in less, unless he's magic.
What Gonchar can do is provide a little supplemental offense on the powerplay, however at this stage in his career I highly doubt that he will be more effective than Nathan Beaulieu. However the powerplay is a very tiny portion of the game, and at even strength, Gonchar looks like a liability.
Where the trade looks bad
Sergei Gonchar is a familiar face for Michel Therrien. He was the top defenseman on the Penguins when he was the head coach there, and Gonchar being injured in 2008-09 is likely the main reason why Therrien was fired, then replaced by Dan Bylsma en route to a Stanley Cup.
Back then, Gonchar was an elite defenseman, similar to Andrei Markov now, and there's a very real possibility that Therrien will see Gonchar as he was 5 or 6 years ago instead of what he is now. As it stands, last season at age 39 only nine NHL defensemen who played 750 or more even strength minutes were given a higher percentage of offensive zone starts than Gonchar was, and only 15 NHL defensemen took a lower percentage of defensive zone starts.
Keep in mind that Dallas has a very, very weak defense, yet still the veteran Gonchar was very sheltered, and the results he gave in those minutes were underwhelming at best, with his team scoring just 46.1% of the goals while he was on the ice, taking 48.2% of the shots, and 48.8% of the shot attempts. The Stars were just under 2% better at controlling shot attempts when Gonchar was off the ice than when he was on it.
To be clear, Gonchar isn't Douglas Murray-level bad, but he's not someone who is going to make many teams better. If the Canadiens are going to make the mistake of putting Sergei Gonchar in the lineup consistently over Jarred Tinordi and Nathan Beaulieu, the team will be significantly worse in essentially every situation.
Where the trade looks good
Travis Moen has some value on the defensive side of the puck, but he is essentially no longer a useful player outside of shorthanded situations. Clearing his nearly league-low possession rate out of the Habs' lineup should help the fourth line be more effective, and likely produce a little more offense.
Moen was making $1.85M this season, and next season, while Gonchar is an unrestricted free against this summer. The Habs have the cap space to accommodate his $4.6M salary (Dallas is retaining $400K) this year, but next year they have new deals to sign with Brendan Gallagher, Alex Galchenyuk, Nathan Beaulieu, Jarred Tinordi, and Michael Bournival. Couple that with the recent stories of the Canadian dollar sinking and the cap possibly not rising, and you can see why Bergevin moved Moen's contract when he could.
If the Canadiens are smart and only use Gonchar as an injury replacement, or a veteran willing to sit in the press box for most of the season while the kids develop, hopefully providing some leadership and guidance, the move is perfect.
Room for optimism?
Two seasons ago Sergei Gonchar put up an unbelievable season for his age in Ottawa, playing semi-sheltered minutes, Gonchar was one of Ottawa's best defensemen. However that was the lockout shortened year, and the difference between 38 and 40 for athletes is, unfortunately, a lot. There is almost no chance that he can recapture that form now.