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 (do this on your desktop web browser; authenticator setup cannot be done via Coinbase mobile app) using your current email, password and 2-step verification method.
Navigate to the security settings page.
Under the Other Options section, select the Select button in the Authenticator App box.
Follow the prompts to complete your authenticator setup.
Follow this important tip for a successful TOTP authenticator setup:
Sign in to your Coinbase account on your desktop browser (not mobile web browser) as your TOTP authenticator requires your mobile device to complete verification.
Note: Coinbase no longer supports Authy. If you're having trouble with Authy, try Google or Duo instead. Enabling an authenticator app will disable SMS code delivery. Disabling your authenticator app will re-enable SMS codes.