Santos Laguna, which reached the finals last year before losing to Mexican rival Monterrey, will host the second leg next Tuesday.
In the moments leading up to the goal, Santos Laguna denied the Sounders a scoring opportunity when goalkeeper Oswaldo Sanchez made a diving save of a shot from the top of the arc by Osvaldo Alonso.
Carlos Quintero quickly started a counterattack, taking the ball down to the top of the restraining arc at Seattle's penalty area. He sent a shot that Sounders goalkeeper Marcus Hahnemann blocked. However, the rebound went out toward the left side of the box, where Gomez ran onto it and parked his shot into the upper right corner of the Seattle net.
The teams met in last year's quarterfinals. Santos Laguna won that series on aggregate, 7-3, falling 2-1 in the opener at Seattle before rolling to a 6-1 second-leg victory at home. Gomez had three of Santos Laguna's goals in that series.
This game was a continuation of the early-season struggles for the Sounders. They're off to a franchise-worst 0-3-1 start in Major League Soccer play with just two goals scored. Their only victory of any kind since opening the league season on March 2 was a 3-1 decision against Tigres UANL of Mexico in the deciding second leg of the CONCACAF quarterfinals at home on March 12.
Santos Laguna had two near-misses on counterattacks in the first half. In the 25th minute, Gomez got behind the Seattle defense and took the ball into the right side of the penalty area. But Zach Scott caught up with the play and forced Gomez to send his shot wide left.
Just two minutes later, Quintero took the ball down the right side and sent it out to the top of the box where Rodolof Salinas one-timed it, but right at former U.S. national team goalkeeper Hahnemann, who made the save.