The Red Devils looked to be heading towards a frustrating goalless draw at St James' Park, until the England skipper closed down what appeared to be a lost cause in the final minute of play and forced the home side's backline into an error to pave the way for Ashley Young's late strike.
Prior to their breakthrough moment, United had come close to breaching Tim Krul's goal on a number of occasions, while up the other end David de Gea was also kept busy and forced into making two big saves.
"I have to say that Wayne, how he makes that goal, is unbelievable, because it was a lost ball. But his fighting spirit is unbelievable," Van Gaal told MUTV.
"David de Gea, this year, is unbelievable. In my career I have a lot of good goalkeepers, for example Edwin van der Sar was one of my goalkeepers and [Victor] Valdes also with Barcelona, but De Gea is having a great season.
"Newcastle is a difficult ground to win at so I am very pleased. They have won here against the top clubs so I am very happy."
The dramatic victory leaves Van Gaal's men sitting in fourth place in the Premier League table.