TOTP Authenticators like Google or Duo provide an extra layer of security in addition to your password.
When using an authenticator for your 2-step verification codes, you'll still be protected even if your password is stolen and your phone number is ported, since these apps are tied to your mobile device, and not your phone number.
When using Google Authenticator, the only way to generate 2-step verification codes is by physically accessing your Google Authenticator app.
How to enable Duo or Google authenticator on Coinbase
Sign in to your Coinbase account using your current email, password and 2-step verification method.