What is observed from Gilman's language choices? How does it look on the page? Why has she chosen this format?

This is an interesting set of questions.  Let us tackle the trickiest one first: some aspects of format are beyond Gilman's control, or beyond the control of any author. She did not, for example, control how the text was presented on the page, what font was used, etc. You might review two or more printings of the story to see what difference this makes.

Turning to the language itself, there are several aspects of the language that are worth noting.

The first, of course, is that it is a first person narrative. Since this is someone who experiences a reality that no one else in the story does, that creates suspense and ambiguity.

Within that first person narrative, there is often a sense that the narrator is engaging someone else--arguing with them, answering them, etc. For example, look at this line early in the story: "Else, why should it be let so cheaply? And why have stood so long untenanted?" To whom is she asking this question?

The narrative is broken into brief paragraphs, creating a sense of fragmentation.

Finally, some of the word choice is striking. For example, consider this early line: "Still I will proudly declare that there is something queer about it." Using the word "queer" indicates the house is strange. Using "proudly" shows she is already emotionally attached, just a few lines in. That's strange.