I'll take a few moments to make the case for Orr here. The biggest factor here will be offence, unquestionably. Gretzky outscored Orr by 66 points. That is significant. Also significant is that Gretzky played in a league that was generally 26% higher in scoring. If Orr's scoring is adjusted 26% upward, he is at 176 points and Gretzky's lead is down to 29 points. Now I am aware that scoring for elite players doesn't exactly mirror changes in league scoring overall, but as a blunt adjustment it isn't entirely invalid. Orr played with an all time offensive great like Esposito, but I'm not sure that his level of support was greater than Gretzky's. Gretzky spend a lot of time playing with Coffey, the second greatest offensive defenceman ever, and most of his forward time with Kurri, an all time great goal scorer who also provided elite defence that partially covered for Gretzky's biggest weakness and freed him to play as offensively as he did. Orr spent a lot of time away from Esposito in addition to his time with him, and it isn't as though Esposito brought something that compensated for a lack in Orr. Both teams were very talented - Bucyk, Anderson, Lowe, and Messier would all become hall of famers, though it's quite possible that only Messier makes it if he didn't play on a team with Orr/Gretzky. each team had a HHOf goaltender. Ultimately I think that neither benefitted all that much from their teammates statistically, in that they would put up astronomical numbers regardless of where they played, but Gretzky probably had a more ideal situation. Gretzky did play in a stronger league.
Continuing from there, Gretzky likely did play in a stronger league. I'm not sure that there was a drastic difference between the NHL of 1971 and 1984, but it would be an edge to 1984. Orr outscored the number ten scorer that season by 83%, Gretzky outscored number 10 by 95%. If you removed teammates from the equation it changes things a little (Orr outscores ten by 93%, Gretzky outscores ten by 116%). Orr's team dominated more at even strength when he was on the ice - Boston outscored teams at over 3 goals to 1 when Orr was on the ice, Edmonton outscored teams around 2 to 1 when Gretzky was on the ice. Each was a huge power play performer, though it probably isn't a big edge for Gretzky - Gretzky didn't dominate power play scoring to the degree he did even strength scoring, with for example Barry Pederson tying him in powerplay scoring and Denis Savard being quite close. Gretzky tied for first in scoring, Orr was second in power play scoring to teammate Esposito. Gretzky was a very useful asset on the penalty kill, scoring 12 goals, but Orr would likely be better as an offensive threat and also an elite defensive player. I'll add that Gretzky missed ~8% of the season, while Orr misses 0%.
In the end I think this comes down to how much someone values an elite defensive defenceman. Orr was still near his peak mobility in 1971 and was elite defensively - in a coaches poll in 1971, Orr tied for first when the coaches selected the best defensive defenceman in the NHL. Gretzky outscored Orr by 66 points that season - that is a significant offensive gap, but Gretzky provided little outside of offence while Orr was an elite defensive player. I suppose you could say that young Gretzky was also a strong possession and transition player, but Orr is possibly the best ever in those areas. Gretzky was playing in a significantly higher scoring league, so the 66 point gap may realistically be smaller than it looks in terms of value. To me I think that a player in the running for best defensive defenceman in the NHL is probably worth 66 (or fewer) points. I get voting for Gretzky, but the argument would have to be derived from a very strong distaste for the NHL of the early 1970s compared to the NHL 13 years later and/or a strong preference for goals.
These players have the two best peaks the NHL has ever seen so I am glad that they go head to head in the finals.