I think with Gretzky we've got 12 world-beating NHL seasons in a row (1979-80 through 1990-91), including four superb best-on-best international performances, and with Orr we've got 6 superb and 2 great NHL seasons in a row, and one very good best-on-best international tournament. So, clearly advantage Gretzky, based on that.
Then, Gretzky has way more hardware, even factoring in certain anti-defenceman biases (like Hart voting, where Orr should have won a couple more).
Then, Gretzky has twice as many Cups as Orr, despite playing in a more competitive era. And three international tournament wins vs. one for Orr.
Based on all this, I think Gretzky gets the nod over Orr without much question.
The negative side for Gretzky is his 1991-92 onward (eight seasons, including two 1/2 seasons). For the first three of those, he's merely great-to-outstanding, but no longer superhuman. And for the next four, he's very good but no longer great-to-outstanding. And for the last one, he's old and done.
But I personally don't hold players' post-prime years against them, as long as the prime years are long enough and consistent enough. Obviously, from 1979 to 1991, Gretzky's prime years are 'high' enough to beat anyone and probably more consistent than any forward ever. 12 seasons in a row, with attendant Canada Cup and Stanley Cup wins, is way more than enough for me -- especially when the player in comparison (Orr) had a shortened career. With Orr, we'll never know what would have been, could have gone either way. We have to just judge on what actually happened.