gakpo is seen as a number 10 (Attacking midfielder) by van Gaal and has a realistic chance of being in the starting squad
I think he could do very well in that position at United as well, but we still have the problem that we have Bruno (and Eriksen) who would arguably favour the number 10 role, so would Gakpo rotate at number 10 for United? Similarly, who would we pick at number 9 out of Martial (injured), Rashford ( better at LW?), Ronaldo (enough said) and Grenwood (enough said). As has been pointed out, using Gakpo as a target man number 9, takes away some of the best attributes of his game.
I am actually in favour of bringing in Gakpo in January and for the rest of the season utilising his versatility as a 9 / 10 / LW / RW, and then bringing in dedicated number 9's, Sesko and Moukoko, in the Summer. I just can't see us bringing in Gakpo now, and then spending another £100m on Osimhen, or £150m on Mbappe, in the Summer, when we have so many other issues to resolve.
If Mbappe is a genuine option, that changes everything, as he is clearly a top 5 in the world, and we would make sure we accommodate him. I can't see why we would bring in Gakpo and Mbappe, who would both want the second striker / LW positions, so if we sign Gakpo, we are not getting Mbappe, and conversely, if we are in for Mbappe, Gakpo will go elsewhere.