Is Gakpo really necessary for United?
Martial - injured 50%
Sancho - badly off form
Pellistri - questionably good enough
Diallo - not good enough
Garnacho - very inexperienced and should be eased into the first team
Elanga - not good enough
That gives you 2 first teamers in Rashford (LW) and Antony (RW) plus 4 maybes in Martial (ST), Garnacho (LW), Pellistri (RW) and Sancho (LW/RW). As has been mentioned many times, you want one or maybe two strikers on the bench who the manager can tactically bring on to change a game. At the moment we have no out and out strikers in reserve, so we are bringing on a winger and shifting Rashford to striker (or starting like that when Martial is injured). 70 minutes into a game, you want to have someone from the bench who can stretch the defence, not a tired out player moving in from the wing.
Whether Gakpo is the right player is a different question. He is not an out and out striker, but is a quality player. I think the rationale is we are better off bringing in a player who will have a long term role to play rather than another stop gap. Let's say we are after a Sesko or Osimhen for the Summer, do we spend lots on a January transfer for Depay or Dembele, who the manager may not be convinced by, or punt on Gakpo, who could turn into a number 9 ala Thierry Henry, or at worst, would be a great second striker to play off Sesko / Osimhen, or to bring on from the bench. Either way, it's not a bad position to be in, so if we can agree a reasonable price, I expect to see Gakpo in January, and a Sesko (my preference as I think he has a higher ceiling) / Osimhen type in the Summer.