Gakpo enhanced his status as one of Europe's best young players with an impressive display at the Qatar 2022 FIFA World Cup.
The 23-year-old scored three goals for Netherlands in Qatar as Louis Van Gaal's side reached the quarterfinals before losing to Argentina.
Gakpo courted interest from Manchester United and Leeds United during the summer transfer window but a move failed to materialise.
His performances in Qatar have reportedly attracted Bayern Munich, Real Madrid and Liverpool to the transfer table and Van Nistelrooy knows it is only a matter of time before PSV lose their prized asset.
"I enjoyed watching him [in Qatar]. He deserves this because of how he has behaved, how he's continued to work and continued to perform," Van Nistelrooy told Voetbal International.
"I was able to help him from very close by and that makes me proud. And then I'll give him his path when it comes.
"If I have the choice, I prefer the summer [window], but it can also be now. You just know there comes a time when you can't say no."
Gakpo has made 24 appearances across all competitions for PSV this season, scoring 13 goals and providing 17 assists.
