Andrew Chambers scored 11 points and Kevin Brashears and Gene Johnson each had 10 for the Gaels (2-0), which shot 18-for-34 from the field.
"It was perfect," said Spierenburg, who was named tournament MVP. "It's a thrill to win this thing. ... Shot selection was a big key. We played great team ball, moved it well and got the looks we needed."
"We played some awfully good basketball in stretches," Robertson said, "and it was different guys making big plays at different times."
Goretti took a 17-10 lead after the first quarter thanks to 9-for-9 shooting from the foul line and the Gaels' stingy defense on Gonzaga, which made just 3 of 11 shots in the quarter.
The Gaels continued to build the lead in the second quarter, scoring 12 of the frame's first 14 points - including two 3-pointers by Kevin Brashears and two jumpers by Spierenburg.
Goretti held a 31-14 lead before Gonzaga closed the half on an 8-0 run - five coming from reserve guard Tramain Gary. Still, the Gaels finished the half 10-for-17 shooting while holding the Eagles to 8 of 24.
"There were a couple bad (shots), but we're learning what shot discipline is all about," Robertson said.
Gonzaga (1-1) cut the lead to six points in the third quarter and five at 41-36 midway through the fourth, but never got closer. Paul Johnson, who finished with a game-high 20 points, fouled out with 2:52 left to play, and the Eagles never got closer than nine points after that.
"When you hold this kind of team under 50 points ... I'm really, really happy," Robertson said.
Chambers and Tim Kellinger were selected to the all-tournament team from Goretti. Paul Johnson was named the tournament's most outstanding player for Gonzaga.
· Northeast Catholic, out of Philadelphia, defeated Bishop Loughlin, out of New York City, 86-79 in Saturday's consolation game.
John Adamski scored 31 points and Andrew Pomager had a triple-double with 27 points, 11 rebounds and 10 assists for Northeast. Both were selected to the all-tournament team.
Devin Ebanks had 19 points to lead Bishop Loughlin, and he was also named to the all-tournament team. Shumuir Williams had 18 points and 10 rebounds and Donald Herndon added 15 points.