People like to undersell Gwen's success by comparing her to Miles and say that she just isn't as good, or comparing her to the original Gwen Stacy. To be quite honest, I think both arguments are a bit silly since they rely on the comparison to say one is bad and the other is good. You can compare Gwen to a lot of other Marvel characters, both from the same time and even before her, that don't have nearly the staying power she has had. Sure, she may be "just" an alternate version of the main Gwen. But that will only last as the new generations grow up and get to know her. For a lot of people already, Spider-Gwen is not "an alternate Gwen" but simply Gwen Stacy, and as cartoons like Marvel's Spider-Man continue to push that narrative, and her silhouette just shadows over the original Gwen Stacy, then people will just have to accept she already surpassed her.
Her success can be attributed to a number of reasons. Honestly. But I think we are underestimating her. Miles was the protagonist of a movie where Gwen was only a secondary character (and she still managed to steal lots of spotlight) and he now has a game incoming. These two factors are rather recent, but have helped to push Miles massively and we could just as easily eventually have something similar with Gwen. While I don't think the Spider-Women movie will be as good as Spider-Verse, or that we won't have a Spider-Man: Ghost-Spider game, its actually far more possible than what I give it credit for.
TL;DR: Gwen's successful.