I guess the issue is that content creators ripped Nintendo assets and included them in their Garry's mod creations, is that right? Hope I didn't misread that.

If so, I'm somewhat surprised that a takedown notice can be so expansive as to include all GM content, and not just individually enumerated creations. I'm sure there are dozens or hundreds of IP violating Garry's Mod creations, but also thousands that don't. If there was just one single example of a Garry's Mod user that ripped a single sprite from a single Nintendo game, would that also necessitate a takedown of everyone's content?