Gressel's goal marked his first in his sixth appearance for Vancouver since arriving in a trade from D.C. United.
It lifted Vancouver (9-11-7, 34 points) back above the playoff line into seventh in the Western Conference. It sits behind the Galaxy since Los Angeles holds the tiebreaker with 10 matches won.
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Sergio Cordova scored his third goal in as many games and team-leading seventh of the season early in the second half for fifth-place Salt Lake (10-8-8, 38 points).
Zac MacMath made three saves for the hosts, including two of exceptional nature after half time, as RSL fell short of their eighth home win.
Gressel is known for his elite right foot as one of the league's best crossers, but scored his first MLS goal since 2021 on a rare goalbound header.
It was a simple goal, with Gauld taking a pass from Ranko Veselinovic on the right side of midfield and bending in a left-footed, inswinging cross.
Gressel had pushed into the box and beat his marker to guide a firm flick on from 10 yards inside the left post.
It was deserved after a flurry of opportunities earlier in the second half.
Just moments after Vancouver's Russell Teibert hit the crossbar, Cordova put RSL in front in the 61st minute.
Jasper Loffelsend received the ball on the right flank near midfield and curled a perfectly weighted early cross toward the arc of the penalty area.
Cordova held his run to remain onside, raced into the path of the pass, cut back to evade defender Veselinovic and then drove a low finish between the dive of goalkeeper Thomas Hasal and the right post.
Both goalkeepers did well to that point. Hasal made an exceptional diving denial of Jefferson Savarino in the 51st minute.
MacMath made a similar stop of a ferocious volley from Gressel in the 59th.