That someone's existence ended up affecting something greatly (even, effecting that thing, period), isn't what we tend to mean by saying "what person had the most impact" on some happenstance.
It's a semantic point, but I think it will hold. G&W certainly did not express any agency in the events claimed to be most significant. Yes, while Mr. G&W was the the thing from which the very plot was made, I think you'd agree that it is only incidental that those things were made from him.
The will of Tabuu was a given beforehand; had Mr. G&W not been useable as a resource to Tabuu in that way, Tabuu would have found some other method of enacting his plot - and, of course, enacting the Plot.
Mr. G&W didn't do anything in having the Primids exist. Their being wrought from his body/essence is not something he did.
And as a separate point, even ignoring the preceding point about agency, I think you can't say G&W had the greater impact on the plot, according to the story proposed on the model of Neo Exdeath and similar, before you say that Tabuu had the greater impact, for Tabuu was in the position of power over G&W absolutely.
So I propose two things: Mr. G&W can't have been "the character who had the most significant on the plot" unless Tabuu, in fact, is the one who had the most significant impact on the plot. (Which is to say, that Mr. G&W definitely did not have the most significant impact on the plot.)
Two, Mr. G&W is not someone who "had an impact on the plot" in any way - at least, in a way resembling the story recently proposed. For I think we will agree that such a thing is not what we would like to mean by a person having an impact on something.