I don't know about if there is a way to do it using the bookdown::gitbook but you could likely do it by adding some custom css to your document.
You can add custom css by including it in the YAML like this:
---
output:
revealjs::revealjs_presentation:
css: styles.css
---
You can then add something like this to your styles.css file:
.page-inner {
width: 95%;
}
I am not sure the class of the gitbook output for the portion of your document you want to make wider. But .page-inner is the name of the div class that is used in the bookdown book so that is a slight chance they are the same. You can find the exact class of this div using the developer tools for your browser.