You and many other say think that Gakpo is the best deal for Manchester United, but I disagree because of one reason and the reason is because Gakpo is a winger just like Anthony, Rashford, Sancho, Martial, Garnacho and Elanga. Manchester United has many of his kind, the difference is that he is younger and reigning for now. My opinion is that Manchester United should go for a full fleshed striker like Victor Osimen of Napoli and Manchester United
About a striker transfer I would support such move for Manchester United also. They just need to be more productive about scoring more goals in a game in the end. A better striker than Rashford and Martial would increase the competition level among them too. Osimhen is just the one for this striker candidate but I don't know if Manchester United can convince even him to come here. Because he is already playing for a much more successful team in the recent times. Napoli are in a much better condition than them this season.
Osimhen's mind might change only if a team like Real Madrid make an offer for him I think. Even then Napoli would demand a lot of money for thinking of letting him go that's for certain.