Right now, the Gmail app for iOS devices only supports Gmail accounts, but the addition of support for third-party email accounts would put the app on par with other popular iOS apps like Spark, Airmail, Alto, Edison Mail, and other options.
Gmail for Android already supports third-party email accounts.
Google is allowing Gmail users to sign up to test the feature through a beta application. Requirements include using the Gmail for iOS app, having a non-Google email account, and iOS 10 or later.
Customers are asked to enter their name, Gmail address, iOS device, and provider of the non-Google email account to be used in the beta. There's no word yet on when the feature might launch following the beta test.
Gmail for iOS can be downloaded from the App Store for free. [Direct Link]