Robin van Persie's early goal proved decisive in the hosts' 1-0 victory but the coach believes there is still room for improvement in attack
Robin van Persie scored the only goal of the match in the fifth minute on the day Van Gaal confirmed his 23-man squad for the World Cup.
But the national coach, who will take charge at Manchester United after the tournament, expressed disappointment at his side's failure to build on that early lead, despite having numerous opportunities to find the net.
"We are too careless with the ball at times," he is quoted as telling SBS 6.
"In the first half, there were many opportunities for us to create opportunities, but we have not taken advantage of them.
"We have created and scored a brilliant goal, but it should have been 3-0. We had another two fantastic attacks."
While feeling his side could have done more going forward, Van Gaal - who is set to employ a five-man defence in Brazil - felt his side were faultless in restricting the threat of Ghana at the other end.
"In the second half we created nothing, but at the same time we gave absolutely nothing away," he added.
"That goes for both the first and the second half and this is the advantage of the system we are using."
Netherlands meet Wales in Amsterdam on Wednesday in their final pre-World Cup warm-up fixture.