Mario had 1 peak season that is on par with Gretzky's 5 best seasons. Maybe 2. But then the best 3 seasons after that are also Gretzky's. Put another way, if we ranked their combined 13 best seasons, 9 of them would be Gretzky's. It would be something like this:
1. Lemieux '89
2. Gretzky '82
3. Gretzky '84
4. Gretzky '85
5. Lemieux '96
6. Gretzky '86
7. Gretzky '83
8. Gretzky '87
9. Lemieux '88
10. Lemieux '93
11. Gretzky '91
12. Gretzky '89
13. Gretzky '81
Peak Mario is arguably equal to peak Gretzky in terms of per-game offensive output, and only for stints of 1 season at a time, but mostly less than that. Gretzky sustained those totals many more times. Sustaining peak totals in hockey is frigging hard! It's tremendously valuable to a team. And that's the huge gaping inescapable gap in your logic that utterly disconnects it from reality.
Emmitt Smith never touched the peak of Barry Sanders. Like, not even close, so that comparison is very bad. Smith's longevity vs Sanders's peak is a good argument because there is a trade off. The discrepancy in both metrics is large.
With Lemieux and Gretzky, there is no trade off. Peaks are, at best (for Lemieux) basically equal, while Gretzky achieved far superior results in the long run.
There is simply no rational basis to have Lemieux equal.