Two late goals from the Netherlands gave them a crucial victory in their World Cup opener.
The PSV forward beat Mendy to the ball when it was thrown to him by Frenkie de Jong, and then nodded the Dutch in front with six minutes left in the game.
The second goal came from a rebound after Memphis Depay's shot hit the post.
Gakpo whirled away in celebration in front of the fans behind the goal as the Dutch supporters, together in a sea of orange, rose to their feet.
They were already chanting their team's name as they knew victory was near, but they didn't know how long it would take.
Papa Gueye had a chance to equalise before Dutch goalkeeper Andries Noppert got his fingertips to the ball.
Both sides lacked ruthlessness in the match. The Netherlands boss risked the fitness of Depay to bring him on as a second-half substitute in the hopes of getting through.
The risk paid off as the Netherlands secured a vital three points in a game that could decide the group's winner.
They are in a tie for the top spot in the group with Ecuador.
Despite missing their main attacking threat, the side can take confidence from the amount of times they troubled the Dutch.
Both sides were at risk of leaving the stadium with a sense of frustration.
Despite an exciting start to the match on the pitch, it didn't last as the game went on.
The African champion had a player on the left in the first half, while the Dutch had one on the right.
The two goalkeepers had been tested by the half-time break. The Dutch who had wasted two glorious chances, the one who took too long to shoot after Gakpo and the one who should have tried himself, the one who was selfish when he should have tried himself.
As the final whistle drew closer, chances fell.
The captain of the Netherlands headed a chance over the bar from a Gakpo corner.
Depay received a huge roar from the Dutch fans when he came on.
His shot was pushed out by Mendy into the path of Klaassen who made it 2-0.
The Dutch did well to come through their toughest match in the group.