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.
Sign in to your Coinbase account using your current email, password and 2-step verification method.
Navigate to https://www.coinbase.com/settings/security
Select the Select button in the Authenticator App box
Note: Coinbase no longer supports Authy. If you're having trouble with Authy, try Google or Duo instead.
Also note that enabling an authenticator app will disable SMS code delivery. Disabling your Authenticator App will re-enable SMS codes.