There is no way they were only average without Gretzky on the ice when they follow up with another 100 point center who was pretty good defensively in Mark Messier who generally had Glenn Anderson on his right side. I think player/line roles and deployment affects these numbers a lot. We have to be missing something.
The 80s' Oilers without Gretzky were certainly (at least in theory) better than average. That is, if the team had been built around, say, Mark Messier as their leadership / #1C / top guy, they would have done very well in the Smythe division and competed for a Cup or two. They might not have got there as fast as they did, and they might not have lasted as long as they did, but I still think they would have got there, if perhaps in a less dominating fashion.
But I think those underwhelming Gretzky-off-the-ice numbers tell us two interesting things (one about the team and one about Gretzky):
1) The Oilers with Gretzky (esp. up to 1986) were not a "normal" team in terms of offensive and defensive strategy. Their tendency, esp. against weaker clubs, was to blow them away in the first half of games (like, taking a 6-1 lead five minutes into the second period) so that the game was effectively over, and then play 'shinny' the rest of the night (giving up all sorts of chances---and thus goals---against). While this didn't much affect wins and losses, it did affect their goals against. A game that for most great teams in history would have ended 4-1 would end 7-5 or whatever, thus ballooning the goaltenders' GAAs and also reducing the goals for/against ratio.
2) Gretzky was not a normal superstar offensive player. What I mean is what Mark Howe said when he described Gretzky as determined, from the drop of the puck to the buzzer at 60 minutes, to get the puck into the other team's net as many times as possible. He never let up, even with one minute left in a game the Oilers were leading 12-2. And as we know, he was effective offensively whether on the ice with Kurri and Coffey or with McSorley and Muni.
I mean, check out the Oilers' plus/minus results here, at the end of January 1987 (after 52 games). There are a lot of good players on this club (mid-dynasty), so how can the plus/minus be this?:
+64 Gretzky (129 PTS)
+40 Tikkanen (55 PTS)
+38 Muni (17 PTS)
+30 Lowe (23 PTS)
+30 Kurri (73 PTS)
+28 Gregg (18 PTS)
+21 Anderson (52 PTS)
+20 Krushelnyski (36 PTS)
+12 Messier (74 PTS)
+12 Coffey (43 PTS)
Obviously, Gretzky was capable of going out and creating goals at even strength by his own initiative.