United won the International Champions Cup with the triumph.
Steven Gerrard gave Liverpool the lead from the penalty spot, before Wayne Rooney, Juan Mata and Jess Lingard scored for United at the Sun Life Stadium.
Van Gaal said afterwards that the victory stands them in good stead for the start of their Premier League campaign.
"This will give us momentum, for sure.
"It's always better to win. First we beat Roma, then we beat Inter Milan with penalties but we were much better, then we beat Madrid and then we had a test against a Premier League team."
United still have one more pre-season game before the season starts - against Valencia at Old Trafford next Tuesday. Liverpool warm-up against Borussia Dortmund at Anfield on Sunday.
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