Gilman's point to me seems a developing technique: one she has recognized but not yet fully realized. Speaking in altered vocalizations is for me a technique for developing a character or narrator's voice features, among other applications, like speaking aloud's value for hearing, tasting words as they might be read aloud and how their flow, pace, and meanings feel, and how accessible and appealing they develop.
I also believe that insights from Mehrabian's communication study apply to Gilman's voice realizations; that is, that "liking" interpersonal communication breaks down to 7 percent of the words actually communicated, 38 percent through vocalized intonation, and 55 percent through nonverbal and nonvocal gestural expression: facial as well as body posture expression. For writers, this implies that since we only have words to express meaning, we ought best pay attention to expressing how vocal intonation and body language inform the meaning of the words.
We often believe our expressed ironies are accessible by those we communicate with in our writing, but as frequently cause misunderstandings.
How to artfully express vocal intonation and body language is a matter of context and texture development. I'm currently reading Henry James' The Beast in the Jungle. Situational and extended ironies comprise a large part of the novel's meaning, and the subtext. James masterfully makes his points and ironies clear and strong, using reflector-narrator commentary to portray and evaluate--artfully and deligthfully mistakenly--vocal intonation and body language.