As already said, what you are referring to is at the browser level, so you would need a browser extension. Or...
create your own Gmail clone and serve it via an apps script web app. As long as there in no malicious code, Google will let you host and serve
your own HTML, CSS, & JS.
For example, I created this so I could quickly see my "must attend" events on my calendar, and simplified the inbox by adding a Archive and Trash button associated with each message. I also only wanted to
see the subject, but can easily view the entire message by clicking the "+" and expanding the section (like an accordion dropdown). I use the "Inbox Zero" philosophy, and I only have 3 emails in my inbox. lol
And since I could do it, I added additional tabs up top for local weather conditions and local county Covid stats.. both just serve an iframe with other apps script webapps embedded.
Yes, you can make your own Gmail client, but you can't fiddle with Google's without a browser extension like Greasemonkey/Tampermonkey.
