Gwen was a non-character written in a one dimensional way. But then you could say that about many characters. Spider-Man, like most comics, was a disposable form of entertainment with a short creative turn around. Gimmicks have always been used to boost sales and Marvel were successful in no small part to their gimmicks. The problem is that one successful gimmick is often reused e.g. every single Marvel superhero early on was misunderstood because people related to it and Stan kept repeating the line about "the world outside your window". So while it was a shocking moment that did have some change, it was also opening the door to a bunch of gimmicks and repetition. And in many ways a lot of comic book stories that are iconic or recommended aren't good per se, but infamous for the cultural change, hence why people think Death of Gwen Stacey is part of the transition away from the Silver Age.