I always felt like this... Wayne been rapping since a kid literally. He don't need nobody to "write" his raps. But the influence undeniable when you look at it from a craft/flow/swag perspective, and less from a "Wayne a superstar, Gillie was a one hit wonder how could he ever write for him" angle.
It was 2 freestyles I seen back in HS (

) that REALLY showed how his influence rubbed off on Wayne during that C1-C2 era (dragging/slurring words with the flow, Calling trucks "Jeeps" on C2(Philly slang))
These the best examples I could find for now.
I think it's like most things. The truth in the middle and got lost in the madness. Like somebody said... When it was cool, Gillie explained it as a coaching, breath control thing. AKA style absorption. But soon as it was smoke, now all of a sudden he wrote everything for the nikka.