Well, I think Gilbert and Gubar tell us that the Author can have his (or in this case her) voice muffled or taken away. Also, I think that it goes to prove Campell's point about rhetoric being confined within external (social, cultural etc) institutions. This applies to the things about female poets from the past that we already know. We know they were mistreated as people and that patriarchal literature structure in a way stifled the voice of these female agents. The fact that this conversation is still a debatable, shows how deeply the roots of "authority" in our literal context run into the culture.
It also shows us that Agency is on a certain level a power struggle. Everyone who thinks they have something important to say wants to get it out there, and for awhile I think we thought there had to be clear winners and clear losers. I believe this because we tried to stifle those we didn't perceive as powerful, while celebrating anything written be someone we thought was great. Just look at Charles Dickens, his work at times is impossible to read (in my opinion) because it is so dull and meticulous. History tells us he made it that way to keep his novels running in magazines longer to make more money. This almost seems like shady business, but we still celebrate him for some reason while the Bronte sisters and I think even Jane Austen take a back seat to Dickens in many people's minds. I think the angst that Gilbert and Gubar describe is the manifestation of women writers desiring to be on a level playing field and having their voice heard (a reasonable request right?)
I imagine that the first female poet wanted to write something great, validating her existence as a poet, proving that women could do it as craftily as a man. I think that besides the dis-ease felt by women poets deciding whether or not to write, there is a dis-ease over "what if I fail" to write something of worth. Which is of course decided by men. (who may not understand where this women is coming from)
Gilbert and Gubar show us that Agency is more complex than just the ability to speak or write persuasively. It is governed by the external as well as what abilities the writer might have. I would contend that it is also governed by the reader/audience. I don't think that anyone hears or is moved by a text the same way because of different experiences, attitudes, cultures, beliefs etc. With each article this concept keeps getting more and more complex to show us how widely an idea can be interpreted.