In addition to the default Mail app on any iOS device, the Gmail client is a standard part of many mobile users' lives. While it's not installed by default, people generally set it up for themselves whenever they activate a new iOS device. So it makes sense that they'd want to use that same Gmail app for their SmarterMail accounts.
Unfortunately, unlike Gmail on an Android device, Gmail on iOS does NOT allow users to set up their accounts using EAS. Shockingly, that includes Office 365 accounts. Whether this is an Apple issue, a Google issue or a Microsoft issue is anyone's guess.
To use the Gmail app for iOS with SmarterMail, you'll want to set up your account using IMAP or POP3.