We need a #9 to supplant Martial as a starter but truth be told, we're best off waiting until the summer. Elite strikers tend not to be available in January and there's no point in us buying a non-elite striker in January unless it's on loan or exceptionally cheap. More significantly, nothing Gakpo has done so far in his career suggests that he can be a #9. He's tall but his back-to-goal play needs a lot of work plus he'll need to fill out if he wants to survive as a #9 in the PL. Also, he hasn't really shown any striker-like movements (Something Martial has regularly been criticised for) so I think spending just under £50M on Gakpo to play him as a #9 seems like a disaster waiting to happen.
Plus whilst he looks a very good player, is he a properly elite player? I think we can argue that we have a lot of good/very good players but very few truly elite players; why add another very good but not elite player to the mix?
Finally, we have no-one behind Martial at #9, pretty much no-one of quality behind Antony at RW (That will hopefully be Amad in time), but we have Rashford, Sancho, and Garnacho at LW. If we sign Gakpo, then we will effectively have 1 ST, 1 RW, but 4 LWs; how does any of that make sense?
Who do we then sell? Should Sancho be moved to #10? Also, we will have effectively used up the guts of £50M from out summer transfer budget on a player we don't really need. We need a new GK, a new RB to compete with Dalot, a new CB to replace Varane in the long-term and likely Maguire in the short-term, a Frenkie de Jong/Enzo Fernandez-like ball-progressing midfielder, a RW to compete with Antony (Could be Amad), and a new ST to supplant Martial. Given all those needs, why would we needlessly spend around £50M on Gakpo?