https://www.theverge.com/2017/10/17/16490972/google-gmail-third-party-email-apps-ios-iphone-yahoo-outlook

Google is testing out the ability to add third-party email services to the Gmail app on iOS. If enabled, it would put the app directly in competition with Apple’s own default Mail app, along with other third-party email apps like Spark and Edison Mail.

Google has offered the option to add third-party accounts on Android phones for a while, which made sense, given that Gmail is the default (and sometimes, only) email app preinstalled on many Android devices. But offering the ability to support other email accounts could indicate a shift in how Google views its iOS app — not only as the best place for accessing Gmail accounts, but as the best email app on the platform, period.