Surprise! It’s someone not connected to Sidney. Karl Gibbs is Sidney and Stab obsessed, and collaborates with others to create a new final girl.
We really don’t know which Ghostface killer is responsible for any of the murders, other than knowing Gibbs is responsible for stabbing Mark Evans.
Ghostface is not immortal but is always hard to kill (hence all the “shoot them in the head” talk). Except Ghostface #1 is mowed over by Gale Weathers’ van, so…
He’s the first killer in the franchise that has no direct connection to Sidney.