In a sign of things to come at Old Trafford, United flexed their muscles as they dismantled the MLS club courtesy of braces from Wayne Rooney, debutant Reece James and substitute Ashley Young in Van Gaal's first match in charge.
Danny Welbeck was also on the scoresheet in United's opening pre-season fixture at the International Champions Cup.
United have at least three games remaining in their tour of the United States, with matches against Roma, Inter and Real Madrid set to follow before opening the 2014-15 Premier League season against Swansea City on August 16.
New recruits Ander Herrera and Luke Shaw were handed starts, while Van Gaal - David Moyes' replacement - opted for a 3-4-1-2 formation that was similar to that used by the Netherlands at the FIFA World Cup in Brazil.
The change in system proved effective during the early exchanges and Welbeck got the ball rolling in the 13th minute, benefiting from an advantage following a foul on Juan Mata before striking the ball off the post and into the net from outside the penalty area.
United gained a stranglehold of proceedings in the 42nd minute, when Rooney converted a spot-kick after the referee awarded a penalty for handball against the Galaxy, who are fourth in the Western Conference standings in MLS.
Rooney put the result beyond doubt on the stroke of half-time, pouncing on some awful defending to bundle the ball over the line for his second of the match.
Van Gaal made wholesale changes at the break, with Darren Fletcher and Herrera the only two players to keep their places in the XI, and it ultimately disrupted the flow of the game.
However, Young cut the ball back in the box and it found its way to James, who scored a similar goal to Welbeck just past the hour-mark, before the 20-year-old struck again with five minutes remaining.
But United were not done there with Young getting in on the action with two goals in as many minutes during the closing stages as Bruce Arena's men were swept aside.
