I personally don't think the changes in goaltender-style/personnel had any effect whatsoever on Gretzky's goal-scoring ability. I mean, had he been 21 in 1999, he wasn't going to score 92 goals, but neither would any other player in history.
Gretzky's reduction in (relative) goal-scoring in the 90s is mainly down to two factors (I mean, besides age):
1) He stopped going to the front of the net and became a perimiter-player
2) He took fewer shots on goal (and those he took were from greater distance because of [1])
Gretzky's shooting percentage in his first twelve NHL seasons (i.e., his prime) varied from 14.9% (the year he scored 215 points!) to 26.7% -- the only time he led the League -- in 1983-84. So, there's an enormous difference already, during his peak years, between 14.9% and 26.7%, and obviously this huge difference has nothing to do with goaltending.
Players Gretzky had a higher shooting-percentage than, in 1991:
Steve Yzerman
Pat Lafontaine
Pierre Turgeon
Sergei Fedorov
Vince Damphousse
I note these players because I don't think anyone accuses them of having trouble scoring on 90s' goaltenders. Gretzky was having better results against 1991 goaltenders than all of them were.
By 1994 (three years into his post-prime and the last time he won the scoring title), Wayne's shooting percentage had dropped to 16.3% (though it was over 17% in late March, before he phoned-in the rest of the season). But how does this compare with his younger peers? Players Gretzky had a higher shooting-percentage than, in 1994:
Pavel Bure
Brett Hull
Joe Sakic
Brendan Shanahan
Theoren Fleury
Kevin Stevens
Doug Gilmour
Even by 1998, when he was considered done as a goal-scoring threat and had one foot in retirement, his shooting-percentage was within 1 percent or so of players like Hull, Yzerman, and Forsberg (and it was better than Fleury, Sakic, Modano).
The main difference was the distance of his shooting and the sheer number of shots. It appears that in the 90s, he averaged around 150-170 shots on net per 80-ish games, whereas in the mid-80s he was putting around 350 shots on goal per season.
(Another point to consider is that, as we know, Wayne's ES-production fell off a cliff after September 1991. Thereafter, he was much more dependent on the PP to supply a lot of his points. And, as you may not know, he had never been aggressive about scoring goals on the PP. In 1984-85, he tallied 73 goals and only 8 were on the PP! Anyway, point being, since more of his points were on the PP after 1991, it follows that his proportion of assist-to-goals would also get bigger.)