Dropping these here for now. My thanks to
@blogofmike and
@The Panther
Those numbers seem cherry-picked to favour Gretzky. Whatever seasons The Panther looked at obviously didn't include 1985-86, when Gretzky played in the same division as the 59 point Winnipeg Jets, the 59 point Vancouver Canucks, and the 54 point L.A. Kings.
I think we need to see the complete picture rather than keep playing dueling anecdata, so I looked up how many games prime Gretzky and Orr had against teams that were below .400 and teams that were above .600 each season and overall during their primes (note: these are all the games that were scheduled, I did not remove games missed by either player as I assume they were distributed randomly):
Edmonton Oilers:
1981: 24 games vs. .600+, 16 games vs. <.400
1982: 9 games vs. .600+, 25 games vs. <.400
1983: 18 games vs. .600+, 12 games vs. <.400
1984: 15 games vs. .600+, 17 games vs. <.400
1985: 14 games vs. .600+, 23 games vs. <.400
1986: 6 games vs. .600+, 33 games vs. <.400
1987: 3 games vs. .600+, 0 games vs. <.400
1988: 11 games vs. .600+, 14 games vs. <.400
640 total GP
100 games vs. .600+ (16%)
140 games vs. <.400 (22%)
Boston Bruins:
1968: 10 games vs. .600+, 4 games vs. <.400
1969: 8 games vs. .600+, 18 games vs. <.400
1970: 32 games vs. .600+, 24 games vs. <.400
1971: 18 games vs. .600+, 24 games vs. <.400
1972: 18 games vs. .600+, 24 games vs. <.400
1973: 11 games vs. .600+, 16 games vs. <.400
1974: 21 games vs. .600+, 15 games vs. <.400
1975: 20 games vs. .600+, 24 games vs. <.400
618 total GP
138 games vs. .600+ (22%)
149 games vs. <.400 (24%)
I also checked extremely bad teams (winning percentage below .300), there were 6 from 1981-1988 and 6 from 1968-1975.
Not really seeing a lot of evidence to suggest that Orr had significantly easier competition. It is interesting that the schedule strength seemed to be a lot more variable in the 1980s, so I do think some of Gretzky's seasons might be a little stronger or weaker than they seem. It is very possible that the main difference between his numbers in 1982 and 1983 was that Gretzky played against better teams in 1983, and Gretzky's numbers in 1985-86 almost certainly aren't as good as they look relative to other years because he crushed the three terrible teams in his division:
Gretzky, 1985-86:
vs LAK/WPG/VAN: 24 GP, 20 G, 54 A, 74 P, +44
vs Rest of League: 56 GP, 32 G, 109 A, 141 P, +27
I also ran the numbers for both Bobby Orr and Bobby Clarke (if there was any other 1970s star up for voting right now I would have picked them instead, but I needed some point of comparison) to see if Orr had unusual splits against the Original Six teams compared to everyone else.
Orr, 1968-1975:
vs O6: 236 GP, 87 G, 213 A, 299 Pts, +163
vs Rest: 324 GP, 159 G, 370 A, 530 Pts, +423
Clarke, 1972-1978:
vs O6: 196 GP, 63 G, 144 A, 207 Pts, +63
vs Rest: 344 GP, 149 G, 330 A, 479 Pts, +277
Orr's PPG is 29% higher against the rest of the league, Clarke's PPG is 32% higher against the rest of the league. Not much of a difference.
Orr's plus/minus per game played is 2.2 times as good as Clarke's against the Original Six, and 1.6 times as good as Clarke's against everyone else, so it is at least possible that the Bruins were better at beating the bad teams by a lot of goals, helping that goals for/against ratio a bit even though Orr wasn't necessarily padding his individual stats (*Edit - my mistake, had that backwards, those numbers actually make Orr look better because he dominated the better teams by more than Clarke did). Ideally I'd like to see a few more points of comparison, as well as to see how Gretzky did against his weaker opponents through his entire prime, but I think I've run enough numbers for today.