The two teams traded 21-point quarters, with Gilroy striking first in just before halftime to take a 27-23 lead at the break.
Seaside, however, put up 21 in the third to take the lad for good.
Leading 44-38 in the fourth, Seaside held Gilroy to nine in the fourth to clinch the win.
Michael Kropff led with 14 points, Connor Doyle followed with nine and Elijah Floyd had eight.
The Mustangs will next hit the hardwood 7 p.m. Friday against Pajaro Valley.
Gilroy wins 2-1
The Gilroy girls soccer team opened the Gabilan Division season with a 2-1 win over San Benito Wednesday
Both teams scored all their goals in the second half.
Gilroy will next travel to Alvarez for a 5:45 p.m. game on Jan. 13.
Christopher falls in opener
Despite goals from Erin Mank and Catalina Martin, the Christopher girls soccer team fell in its league opener 4-2 at Salinas.
The Cowboys tallied a pair of goals in the first half and held off a charge by the Cougars for the win.
Salinas scored twice in the second half for the final margin of victory.
Christopher suffered its first loss of the season, falling to 6-1-1 overall.
The Cougars will next host Monte Vista Christian 5:45 p.m. Friday.