Wayne Rooney, Ashley Young and debutant Reece James all scored twice, and Danny Welbeck also found the net as United crushed the Galaxy in the Pasadena Rose Bowl.
Ander Herrera, also turning out for United for the first time, showed why the club paid £28million for his services.
The Spaniard put on a dynamic and destructive display in Los Angeles, breaking up play and spraying killer passes across the park.
Van Gaal, who sent United out in the 3-5-2 system he used with Holland at the World Cup, rejected an offer to play down the importance of the win, insisting his new squad should take great confidence from dishing out such a hiding.
"The result always matters
With such a result you get confidence," the new United boss said.
"It is fantastic how they have performed today.
"When you see us train, you can expect something but not 7-0
It was a surprise, but they were beautiful goals and beautiful attacks."
Welbeck got United off to the perfect start with a 25-yard finish and Wayne Rooney made it 3-0 after a penalty and an easy tap-in following a glaring error by Tommy Meyer.
James, who was one of nine half-time substitutes, tucked away two tidy finishes from inside the box and Young then rounded off the rout with two late goals.
Many had suggested Van Gaal did not have the strength in depth to send United out in a 3-5-2 formation, but he had no qualms about doing so in California.
The 62-year-old, who used the formation for most of his run to the World Cup semi-finals in Brazil, deployed Luke Shaw and Antonio Valencia as wing-backs while Jonny Evans, Chris Smalling and Phil Jones started at centre-half.
With Darren Fletcher, captain for the night ahead of Rooney, and Herrera protecting the defence, United looked solid all night.
It was a surprise, therefore, to hear the squad had barely practiced playing in that formation.
"We introduced a new system that we have only played two times on the training pitch 11 v 11," the former Barcelona manager said.
"But the boys are willing to pick up the information.
"It is fantastic how they have performed.
"Last week, 10 players who played tonight were not even in our training complex.
"When you want to change a system, you must start at once
We don't have time to prepare for other things.
"The other system they can play is 4-3-3 and they have played it for many years.
"I can change back if the system doesn't work
With the quality of the players we have, I can play 4-3-3 with three strikers on the bench, but I want to play with two strikers."
Van Gaal's predecessor David Moyes usually played 4-2-3-1 as his squad was awash with players who like to play just behind a main striker.
Van Gaal believes the Scot left him with an "unbalanced" squad.
He said: "We have four number 10s, so the selection is not balanced in my eyes.
"I have decided to play this system because of the quality of the players
But if we lose, I can change back to another system."
Galaxy coach Bruce Arena thinks Van Gaal's introduction has sharpened the United squad up following their seventh-place finish last year.
"I have nothing but the greatest respect for the show that Manchester United put on," he said.
"A bunch of guys out there fighting for jobs and it looked that way
"It was a real lesson for a lot of our players."
Source : PA
Source: PA