I think you can actually add other providers to Gmail, but my comment was about how most people I know only use Gmail, and they have no need to configure their os-of-choice's mail app. I guess that was kind of implied. Sorry if I was ambiguous!

I might be an anomaly. I've ran iPhones before and have an iCloud email address presumably, but have only ever used the Gmail app for email on my iOS devices. And on my other machines I just use the web app. This seems to be the same for most people I've worked with in the last 5 years or so. Obviously it's super anecdotal and just from observation.