Erase autofill email addresses in gmail on iphone 6

if there is a (i) or similar icon against the address tap on the icon and select delete recent

its probably a Gmail Other Contacts thing and you may need to sign into Gmail on a laptop

https://support.google.com/mail/thread/3884163?hl=en

Whether it is Mail app or Gmail app you are using.

Delete the gmail accounts

Delete the app(s)

Reinstall app(s)

Add back accounts

That’s Google for you, like trying to scrape gum off your shoe.