The Whitecaps (12-9-13, 49 points) needed a win or draw in the regular-season finale to qualify for the postseason. Goalkeeper Maxime Crepeau helped assure that by making three saves, including one in the final minute of second-half stoppage time, as Vancouver finished with a six-game unbeaten streak (3-0-3) since a 4-1 loss Oct. 9 at Seattle.
Fredy Montero scored on a penalty kick for the Sounders (17-8-9, 60 points), who could've clinched the Western Conference title and a first-round bye with a victory.
Instead, Seattle, which opened the season with an MLS-record 13-game unbeaten streak, finished with a six-game winless run (0-3-3) and dropped into second place behind Colorado, a 5-2 winner against Los Angeles FC on Sunday.
The Sounders took an early lead, with Montero converting a penalty kick in the eighth minute. A corner kick by Nicolas Benezet for Cristian Roldan went off the arm of Vancouver's Cristian Dajome for a penalty.
Montero, who played his past three MLS seasons in Vancouver after starting his career in Seattle, hammered the kick into the lower right corner of the net as Crepeau went the other way.
Gauld tied it in the 20th minute. A cross from the left wing went off the head of Vancouver's Brian White just before he collided with Seattle defender Nouhou Tolo and to Gauld near the top left of the 6-yard box. Gauld put a header off the far post and into the net. White suffered a gash over his right eye on the play but stayed in the game after being attended to.
Crepeau, a Canadian international, made three saves in the first half to keep the Whitecaps close. He stopped a shot from outside the penalty area by Seattle's Kelyn Rowe in the 18th minute, a point-blank attempt by Montero in the 26th, and a 6-yard shot by Roldan in the 34th.
Crepeau also smothered Montero's attempt from 5 yards out in second-half stoppage time.