I've probably watched every professional game Gakpo has played and as much as I like him, I don't think he's a great fit here.
He's without any doubt the best and most complete player in the eredivisie right now. He is very calm and collected in front of goal. He can finish with both his left and his right. He's physically strong and can hold a ball. He has a great whipping cross.
However he is at his best when he has a little space or a second to consider his options. In tight spaces he isn't the best creator. I think his long term future will be as a CF but he's hardly ever played there, I can't recall a single match where he did. I think that of he goes to United now he will flop. As long as there's no creative midfielder to send him deep he won't reach those 15+ goals that make him so good. In the prem he won't have the time he needs for his crosses. He needs an intermediate step, like most eredivisie players when going to the prem.
Also I really want him to stay at PSV for this season but that's unrelated to my analysis. Minor conflict of interest that I dod have to mention, although I love PSV players going to United.