Players are most often 'minuses' when their teams lose in the playoffs -- esp. if the teams lose in four or five games. Generally, if your team loses four out of five games and you're on the ice a lot, you're a minus. Rather than looking at individual playoffs years (which compares a 1-round loss to a 4-round Cup win -- guess which plus/minus is going to look better?), it might be instructive to look at overall career playoff results at even strength, particularly for players who had long careers on diverse-level clubs (i.e., good and bad clubs, not only dynasties).
In Gretzky's case, his career playoff plus/minus works out to +92 in 208 games, or +36 per 82 games.
In Guy Lafleur's case, his career playoff plus/minus works out to +50 in 128 games, or +34 per 82 games.
In Mario Lemieux's case, his career playoff plus/minus works out to +20 in 107 games, or +15 per 82 games.
In Jaromir Jagr's case, his career playoff plus/minus works out to +30 in 208 games, or +12 per 82 games.
In Bryan Trottier's case, his career playoff plus/minus works out to +31 in 221 games, or +11 per 82 games.
In Gilbert Perreault's case, his career playoff plus/minus works out to -21 in 90 games, or -19 per 82 games. (Not surprisingly, he's the only guy here who didn't win any Cups.)
(Bobby Orr, not unusually, stands out here. He works out to +66 per 82 games. It's incredible, but of course it's an "All killer-no filler" playoff career, entirely on good teams and in his prime years.)