Answers
Based on "The Yellow Wallpaper", I do not consider Gilman to be a muckraker.
Explanation:
"The Yellow Wallpaper" is more a psychological/social exploration through a feminist lens than anything else, I think. It was a response to the "rest cures" for psychosis recommended by the doctors at the time.
I don't consider it muckraking because, while it is based off of her own experiences, it is not about any actual one incident. Muckraking uses investigative journalism to create real narratives that bring attention to an issue through shock value. Nelly Bly was a muckraker because she exposed specific injustices that she witnessed in mental hospitals.
Gilman does discuss injustices, but she does it through the filter of fiction. She does not directly say "while on rest cure, I experienced x, y, and z".
She instead uses a fictional character to reflect her experiences, and "the female experience" in general, and the injustice involved, which is fine, but I don't consider it muckraking.