As it stands right now he isn't a #9.
That doesn't neccesarily mean we shouldn't sign him though. We do need a proper #9, but it might be we'd plan on getting that #9 in the summer and just making do with Martial/Rashford/Gakpo up front until the end of the season. Which isn't ideal, but is understable if the CFs we really want aren't available until the summer.
The question for me is if we assume we'll be starting next season with a new #9, do we still want Gakpo? And it might be that we do, depending on what we see as his long-term position. Or indeed how much faith ETH actually has in someone like Sancho, say.
Completely hypothetical but let's say we started next season with these attacking options:
CF: New CF, Martial
LW: Rashford, Gakpo, Garnacho
RW: Antony, Sancho, Elanga
It doesn't particularly seem like overkill to me, especially with a few of those players being able to play in different positions.