But i have not login my gmail yet so why this error displayed me?
This means that the gmail you’re trying to use to log in\register is already connected to an acount, but google authentication is not in use on that account.
You’ll need to either log in with your password, or do a password recovery
Because clearly that’s not the case. The error message already said it: the email address is already registered.
If you don’t know the password, or you don’t know which login method is used, please request a password reset for your email address instead. Either you’re receive a link to configure a new password, or receive information about which login method you can use instead.