I think the biggest problem is that it's not just the gangbang, but how it is described in detail, to the point of feeling like it's stephen being a pervert rather than a unsettling scene
There's lots of sexual stuff involving children in King's work. I'm not sure about the last 10 or so years of his stuff but in the 80s/90s he could barely go a book without including something along those lines.
I think in recent years he's said he wouldn't have included stuff such as the group sex scene from
IT because he feels culturally we have shifted and he no longer things those passages are viewed in the same way given the different context those things have for society now after all the Catholic sexual abuse was uncovered, greater widespread understanding of the consequences of sexual abuse, and other things along those lines that have gradually pushed people's thinking on the matter.
His goals was for that stuff to read more like "wow that is really warped" but now it's seen more as "goddamn this writer is a fucking sick bastard in the worst way, why isn't he locked up yet?"