Loyola's Jamie Dubyoski collected a pass from John Jlopleh just outside the penalty area and had a spilt second to decide where to place his shot against Gilman goalie Andrew Harris.
Dubyoski saw that Harris was not charging off his line, so he quickly slotted the ball just inside the near post with 21 minutes remaining.
The fourth-ranked Dons then continued to attack and held off a late rally by the No. 1 Greyhounds for a 1-0 boys soccer victory in Roland Park.
Doing so, the Dons created a log jam atop the Maryland Interscholastic Athletic Association A conference, where they sit atop the league with one loss along with Gilman, Archbishop Curley and McDonogh.
"We've played Harris for three years now, and I've always gotten lucky against him," Dubyoski said. "He's an outstanding goalie. It's always tough mentally when you're going against a top-tier goalie like that. We just really had to take our chances, and we did."
Harris, last season's Baltimore Sun All-Metro Player of the Year, who will attend Wake Forest, came within a few feet of tying the game in the closing minutes when he fired a shot from half field. But the ball went just wide and the Dons were able to control possession the rest of the way.
Dubyoski created numerous chances for himself in the second half by making effective runs off the ball. The Dons (6-1-1 overall, 4-1-1 MIAA) spent most of the week in practice creating space on the attack with diagonal runs, and the work paid off against Gilman (9-2 overall, 6-1-1).
Loyola goalie Sam Hanke made six saves and knocked away several dangerous.
"The strength of our team is to be able to keep the ball and be a little bit fluid," Loyola coach Lee Tschantret said. "We did a good job today, but I think the key today was keeping a clean sheet. We did all of the little things well."
Gilman had several chances in the first half, but could not convert. Jake Matthai, a 6-foot-4 forward, had several dangerous headers that narrowly missed.
"We knew Loyola was a great team, and you have to give them credit," Gilman coach Jon Seal said. "Coming in, we said we couldn't let Dubyoski be a factor, and he scores the game-winner. I'm disappointed we let this one get away."
No. 4 Loyola 1, No. 1 Gilman 0
Goals: L-Dubyoski. Assists: L-Jlopleh. Saves: L-Hanke 6; G-Harris 8. Half: 0-0